Fiber optic lasers and amplifiers

ABSTRACT

A fiber optical source of stimulated optical radiation comprises an optical fiber which includes a core doped with laser material having optical gain in two wavelength regions, the fiber additionally including a material in optical communication with said laser material in such a manner as to absorb radiation emitted from said laser material within one of said wavelength regions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 815,741, filed Jan. 2, 1992,now U.S. Pat. No. 5,363,463, which in turn is a continuation-in-part ofapplication Ser. No. 441,942, filed Mar. 12, 1990, now U.S. Pat. No.5,096,277, which in turn is a continuation-in-part of application Ser.No. 293,119, filed Jan. 3, 1989, abandoned, which in turn is acontinuation-in-part of application Ser. No. 102,835, filed Sep. 30,1987, pending, which in turn is a continuation-in-part of applicationSer. No. 711,062, filed Mar. 12, 1985, now U.S. Pat. No. 5,004,913,which in turn was a continuation-in-part of application Ser. No.608,932, filed Mar 14, 1984, now U.S. Pat. No. 4,708,494, which in turnwas a continuation of application Ser. No. 405,732, filed Aug. 6, 1982,now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to new fiber optic lasers and amplifiers and tomethods and devices for the remote sensing of physical variables withfiber optic systems, and more particularly to new sensing techniquesbased on optical power distribution between two light-guiding regions ofan optical fiber probe.

DESCRIPTION OF THE PRIOR ART

Fiber optic sensing systems have been under development in recent yearsfor the remote measurement of physical variables (also referred to asmeasurands) of interest in industry, medicine, transportation, and otherfields of human endeavor. There are two main approaches to fiber opticsensing, as follows:

(a) The use of the optical fiber itself as the transducer, in a systemwhich may be interferometric or non-interferometric, and

(b) The use of a point transducer attached or otherwise opticallyconnected to one or more optical fibers, the latter acting merely aslight guides.

These and other approaches are discussed in a recent review by G. D.Pitt et al., IEE Proceedings, Vol. 132, Pt. J, No. 4, August 1985, pp214-248.

In addition, optical fibers are used for transmitting information ofsensors, whether optical or electrical, to a remote control station.

Interferometric sensing systems use the fibers themselves astransducers, and can achieve high sensitivities. They have, however,some disadvantages. They usually require single mode lasers, single modefibers, and relatively complex instruments, and are often subject todrift and phase noise due to environmental factors other than theparameter sought to be measured. Further, they are not presentlycompatible with the industrial requirements for ruggedness.Non-interferometric systems, which usually use multimode fibers andnon-coherent light sources, are simpler, more rugged, and are capable ofmeeting most of the sensitivity requirements of industry. The methodsand devices of this invention are based on non-interferometric systems.

Many fiber optic sensing systems are based on light intensity changesproduced by the action of the measurand. In order to obtain a reliablemeasurement of the physical variable with such systems, it is necessaryto compare the optical signal generated by the measurand to a referencesignal which is not affected by this measurand. This is done in theprior art by any of the following ways:

(a) If the sensor is not spectrally selective, by splitting the lightoutput from the interrogating light source into two separate beams eachcarried by a separate fiber. One of the beams is made to interact withthe measurand, where its intensity is modulated according to themeasurand's value. The other beam is used as a reference. The modulatedbeam and the reference beam are sent through separate optical paths toseparate photodetectors, and the resulting electrical signals arecompared. The value of the measurand is determined from the relativevalues of the two electrical signals.

(b) If the sensor is spectrally selective, by interrogating the probewith light of an intensity distributed in a known manner between twowavelength regions, the intensities of each of these spectral componentsmodulated in a known, different manner by the probe. There are two waysto accomplish this. One way is to carry both spectral componentssimultaneously by the same fiber, and then to separate them at the fiberoutput (after interaction with the sensor) by means ofwavelength-selective optical filters, feeding each separate beam to aseparate photodetector. The other way is to use a switching device tosend both spectral components alternately through the same fiber systemto the same photodetector. The latter method was used to measuretemperature by monitoring the temperature-dependent light transmissionof semiconductor crystals within their absorption band edge (Kyuma et.al., IEEE J. Quant. Electron., QE-18(4), 677 (1982).

Each of the above methods is subject to error, due to unequal detectordrift, varying losses in the separate optical paths and/or unequalfluctuations in the intensities of the spectral components of theinterrogating light source(s).

In the prior art sensors described above, so-called intensity sensors,the value of the measurand is determined indirectly from the attenuationof the intensity P_(o) of the interrogating light incident on the sensorto a transmitted value P_(o) (1-α). The value of α, which is anindicator of the value of the measurand, is estimated indirectly fromthe measurement of the transmitted light intensity. It is well knownthat such measurements can not be made with a high degree of accuracywhen the optical density is of the order of 10⁻³ or lower for a single,discrete sensor. The accuracy of said attenuation measurements isfurther degraded in the case of series-multiplexed sensors, where theattenuation measurements must be carried out by optical time domainreflectometry (OTDR) techniques, which consist of the measurement of thedecrease of the intensity of the Rayleigh-backscattered light caused byeach sensor. In this case a measurement with an accuracy of one percentmay require the accurate and reproducible measurement of light intensitychanges of a few parts per 10⁴. This is difficult to achieve with arelatively simple device, especially considering thatRayleigh-backscattered light signals are relatively weak, with anintensity down about 45 to 50 dB with respect to the intensity of theinterrogating light at the same fiber location.

Most non-interferometric fiber optic sensing systems operate bymeasuring the measurand-dependent variable light transmission of thesensor, and their performance, is subject to the inherent limitationsdiscussed in the preceding paragraph. Temperature-sensing systemsinclude, besides the one already mentioned by Kyuma et. al., the systemssubject of U.S. Pat. Nos. 4,136,566 (Christensen), 4,302,970 (Snitzeret. al.), 4,278,349 (Sander), 4,307,607 (Saaski et. al.), and thecryogenic temperature sensor described in NASA Technical Briefs, p. 55,Spring 1981. All of these systems are based on a temperature-dependentlight attenuation.

Other sensor systems based on variable light attenuation and designed tomeasure a variety of measurands include the ones described in U.S. Pat.Nos. 4,356,448 (Brogardh et. al.), 4,433,238 (Adolfsson et. al.), and4,523,092 (Nelson). There are many others, but they fall within the samecategory of attenuation sensors covered by these references.

Fiber optic sensing techniques are especially suited for distributedsensing and for the multiplexing of a multiplicity of discrete sensors,and considerable interest has developed in recent years in theseapplications. A comprehensive review has been published recently (J. P.Dakin, J. Phys. E: Sci. Instrum 20, 954 (1987)), which discusses manyapproaches suggested or under development for these purposes. Some ofthese methods and approaches were first disclosed after the filing ofthe original application (Ser. No. 405,732) of which this is aContinuation in Part. OTDR techniques seem to be most extensivelyinvestigated, mainly coupled to the measurement ofRayleigh-backscattered light, but Raman and fluorescence variants havealso been proposed. An anti-Stokes Raman technique has been proposed formeasuring temperature (Dakin et. al., Electron. Lett. 21 569 (1985))based on the temperature-dependent occupancy number of a vibrationallyexcited level in a glass. The method provides signals which are ordersof magnitude weaker than a method, also based on a temperature-dependentoccupancy number of a vibrationally excited molecular level, subject ofsaid original application Ser. No. 405,732 and using fluorescenceconversion, and can be regarded as a less sensitive variant of saidearlier invention. A recently proposed fluorescence technique proposedby Dakin, mentioned in said review article, is based ontemperature-dependent changes in the fluorescence spectral distributionof some fluorescence dyes, in contrast to the methods of this invention,which do not require any temperature-dependent change in anyfluorescence property and can, therefore, be implemented with most dyes.

A method for measuring distributed forces has recently proposed byFarries and Rogers and discussed in another review article (A. J.Rogers, J. Phys. D: Appl. Phys, 19 2237 (1986)), based on the effect offorce-induced changes of the polarization properties of a single modefiber on the stimulated Raman gain, under intense optical `pump` pulses,from sensing points along the optical fiber. The method has thedisadvantage (among others) that polarization changes at one point alongthe fiber affect the polarization properties of the fiber along the restof its length, and the results are thus difficult to interpret.

Macedo et. al. (U.S. Pat. No. 4,342,907 describe an interestingOTDR-based system for the measurement of distributed forces acting atdifferent pre-determined points along a fiber optic cable. These forcesdivert a fraction of the intensity of the interrogating lightpropagating along the fiber from the fiber core into the cladding. Thisdiverted light is removed by a coupler mechanically attached to thecable at each sensing point and transferred either to a second fiberoptic cable (the return light guide) by means of another coupler, whichdirects this light to a photodetector in the OTDR device, or reflectedback by an external detector into the same fiber optic cable towards theOTDR device. While the system has the advantage that it produces opticalsignals which are a linear function of the intensity of the light forcedout of the core, and are free of the baseline background of theinterrogating light intensity, it is mechanically and optically complex,requiring at least one directional coupler per sensing point, and isunsuitable for detecting or measuring forces at point other thanpre-determined ones.

It is an object of the present invention to provide methods and deviceswhich eliminate or minimize the sources of error discussed above, saidmethods and devices producing both a single beam and a reference beamfrom a single light source, whether this be a broad band or a narrowband or monochromatic source, carrying both beams simultaneously througha single fiber to a single photodetection station, and separating,measuring and ratioing by simple means the electrical signals generatedat the photodetection station by both the signal beam and the referencebeam.

It is another object of this invention to provide a simple method formeasuring directly and accurately small attenuations of an interrogatinglight beam produced by the action of a measurand, by generating a signallight intensity proportional to said attenuation and free of theintensity of the interrogating light incident on or transmitted by thesensor.

Still another object of the present invention is to provide new methodsand devices for the measurement of diverse measurands at a plurality ofremote locations simultaneously or quasi-simultaneously, using a singleexcitation light source and a single photodetector, with the individualprobes attached to a single unbroken optical fiber, said measurementsbeing only minimally affected by fluctuations of the intensity of theinterrogating light beam, fiber losses or detector drift.

Yet another object of the present invention is to provide a longunbroken optical fiber as a distributed sensor, said fiber beingsensitive to both mechanical forces and temperatures acting at differentpoints along the fiber, and associated devices for the accuratemeasurement of the location and magnitude of said distributedtemperatures and mechanic forces.

Other objects of the present invention will in part be apparent from thefollowing discussion and will in part appear hereinafter.

BRIEF SUMMARY OF THE INVENTION

These and other objects are accomplished by using so-called intensitysensors in a new manner whereby, in a preferred embodiment of theinvention, interrogating light is launched into the core of an opticalfiber probe for a measurand, wherein a fraction α of its initialintensity is removed from the core, the value of α being an indicator ofthe value of the measurand. Instead of discarding this removed fractionas in the prior art and estimating its value indirectly from the lightintensity P_(o) (1-α) transmitted by the probe, the removed light iscaptured and processed into an optical signal separable from thetransmitted interrogating light and from signals from measurands actingsimultaneously at any other point(s) on the fiber, but carried by thesame fiber to a photodetection station, so that both the intensities ofthe transmitted interrogating light light beam and that of said removedfraction α, the value of α being an indicator of the value of thephysical parameter, can be measured to give a ratiometric (referenced)reading of the value of the measurand, unaffected or only minimallyaffected by fluctuations of the intensity of the interrogating light,fiber losses or detector drift. The processing of α into a separable,directly measurable signal, can be effected either in the opticalspectral domain, by luminescence or Raman conversion into a light ofdifferent wavelengths from those of the interrogating light, or in thetime domain, using pulsed or AC-modulated interrogating light andprocessing said fraction α into a signal which arrives at thephotodetector either at a different time or with a phase shift relativeto the interrogating light. An important feature of the invention isthat the sensing materials it uses can be homogeneously incorporatedinto components of long optical fibers allowing the measurement oftemperatures and/or forces distributed over many locations,simultaneously and with a single fiber probe.

In accordance with the above description, the invention comprises themethods and devices for processing said fraction α into said separableand directly measurable signal, including the novel optical fibers whichare necessary and essential for making said methods and devicesoperable, and the application of said methods and devices for thesensing of distributed physical parameters using a continuous length ofone of said novel optical fibers as the sensing probe. The methods anddevices for sensing said distributed physical parameters use timedivision multiplexing (TDM) techniques, including both theback-scattering TDM techniques better known as "optical time domainreflectometry" (OTDR) techniques used in most of the distributed sensingsystems of this invention (including the backward-stimulated lightamplification and the Raman back-scattering techniques, which do notusually measure reflected light), and the `forward` TDM force-sensingtechniques described in section 3.5. The most common TDM techniques arethe OTDR techniques. Accordingly, the claims of this application aredrawn to the novel optical fibers which are necessary and essential formaking said techniques operable, and to the OTDR and other TDM methodsand devices suitably modified according to the teachings of theinvention.

In contrast to prior art sensing methods using luminescent materials,the luminescence conversion techniques of this invention do not requireany change in the luminescence spectral distribution, quantum efficiencyor decay time of the sensor materials.

DEFINITIONS AND SYMBOLS

Within the context of this application, I am using the followingdefinitions:

Light: optical radiation, whether ultraviolet, visible, or infrared.

Vibronic material: any material whose molecular electronic ground levelcomprises a plurality of vibrational sublevels with energies higher thanthat of the lowest occupied level of the material.

Vibronic level: a vibrational sublevel of the electronic state ground ofa vibronic material, having an occupancy number which increases withincreasing temperature.

Luminescence converter: a material which absorbs light of wavelengthsλ_(s) and emits at least a fraction of the energy of the absorbed lightas luminescence light including wavelengths λ_(f) different from λ_(s).

Raman converter: a material which, when exposed to light of any photonenergy E_(s), converts a fraction of the intensity of said light intolight of a photon energy different from E_(s) by an amount approximatelyequal to the energy of a vibrational quantum of said material.

Occupancy number of an energy level (or sublevel): the fraction of themolecules or ions of the material occupying said energy level (orsublevel).

Population inversion: a condition in which the occupancy number of anexcited level is greater than that of a lower level to which themolecules or atoms of the excited level decay through a radiativetransition.

Measurand, physical variable or physical parameter: any physicalproperty whose magnitude can change. Examples: temperature, force, flowrate, level, position.

Interrogating light: light launched into a sensing system for thepurpose of generating an optical signal indicative of the value(magnitude) of the physical parameter being sensed or measured.

Excitation light: interrogating light which generates a luminescence orRaman-shifted light of an intensity determined by the magnitude of themeasurand.

Force: any action which, on an optical fiber or other optical probe,affects the transmission of light along it. Examples: stress, pressure,sound waves.

λ_(v) : a wavelength or wavelengths of interrogating light at which theabsorption coefficient of a sensing probe is temperature-dependent.

λ_(f) : a wavelength or wavelengths of light generated at a sensingprobe by interrogating light of wavelength λ_(s) and different fromλ_(s).

α: the fraction of the intensity of interrogating light attenuatedwithin a sensing probe at a sensing point under the action of ameasurand.

λ_(s) and α_(v) : fractions of the intensity of an interrogating lightof wavelengths λ_(s) or λ_(v), respectively, attenuated within a sensingprobe at a sensing point.

Effective optical path length of a light guide: the product of theactual path length of the light guide times its index of refraction.

Cladding: a light-guiding region surrounding a fiber core, whether ornot in physical contact with the core and having a thickness no smallerthan that of the shorter wavelength of optical radiation transmitted bypure silica, and regardless of the value of its index of refraction.

Proximal end (first end) and distal end of an optical fiber: the endinto which interrogating light is launched (injected) is the proximalend. The other end is the distal end.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an energy flow diagram, at the molecular level, of a class ofluminescent materials useful for measuring temperature according to theinvention.

FIG. 2 shows the temperature dependence of the normalized fluorescenceintensities of three organic dyes useful for measuring temperatureaccording to this invention.

FIG. 3 is an energy flow diagram, at the molecular level, of anotherclass of luminescent materials useful for measuring temperatureaccording to this invention.

FIG. 4 illustrates a distributed optical thermometer according to thisinvention.

FIG. 5 illustrates the principles of anti-stokes Raman thermometry.

FIG. 6 illustrates another optical fiber probe for the measurement ofdistributed temperatures, having a central core with atemperature-dependent numerical aperture.

FIGS. 7 and 8 describe two optical fibers useful for practicing thisinvention, having a luminescent cladding.

FIGS. 9, 9A and 9B illustrate an arrangement for the measurement ofdistributed forces using optical fibers having two light-guiding regionsof different effective optical path length.

FIG. 10 represents a device for measuring distributed forces andtemperatures according to this invention, using a single continuousoptical fiber probe.

FIG. 11 shows a system for measuring distributed parameters on a singleoptical fiber using an AC-modulated C.W. light source, by phase angledivision multiplexing.

FIG. 12 illustrates the principles of distributed sensing with singlelong fibers using optical `pumping` and amplification ofcounterpropagating light.

FIG. 13 shows a distributed thermometer using a ND(III)-doped glassoptical fiber.

FIG. 14 illustrates a device for measuring distributed forces andtemperatures with a single long optical fiber probe, using stimulatedlight amplification processes.

FIG. 15 illustrates a twin-core fiber suitable for sensing distributedforces and temperatures according to the invention.

FIG. 16 illustrates an optical fiber probe for sensing distributedphysical parameters by backward-stimulated Raman scattering processes.

FIG. 17 illustrates the power distribution of the lowest order modesbetween the core and the cladding of an optical fiber, relative to thedifference Δn of their indices of refraction.

FIG. 18 illustrates a two-dimensional tactile sensor according to thisinvention.

FIGS. 19, 19A and 19B illustrate fiberoptic keyboards according to thisinvention.

FIGS. 20 and 21 illustrate sensor fibers suitable for concentrating theoptical powers of `pump` light sources into fiber cores ofcross-sectional area smaller than the emitting area of the light source.

FIGS. 22-23 illustrate arrangement for the pumping of Er(III)-dopedfiber lasers.

FIG. 24 illustrates a light-diffusing photo-irradiation probe formedical applications, capable of measuring its own temperature.

FIG. 24A illustrates a system for laser angioplasty with atemperature-sensing `hot tip`.

FIG. 25 is a representation of a system for the sequential multiplexingand transmission of signals from electronic sensors on a continuousoptical fiber length.

FIG. 26 illustrates a fiber optic bus-organized system for sensor dataacquisition, using the fiber of FIG. 9.

FIG. 27 shows an infrared image conversion film according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

1.0. The Direct Measurement of Small Optical Attenuations Caused byPhysical Variable

In fiber optic sensing technology, optical attenuations areconventionally measured by indirect techniques. As the following exampleshows, such approach is not suitable for the measurements of smallattenuations. Suppose that one is measuring temperature with an opticalprobe characterized by a temperature-dependent optical absorptioncoefficient at a wavelength λ_(v). The optical density of the probe islow enough that, at a temperature of 300 kelvins (I), the probe absorbsa fraction α_(v) equal to 0.0100 of the power P_(o) of the interrogatinglight at that wavelength. The fraction α_(v) is not measured directly inthe prior art in optical sensors. What is measured is the power P of thelight transmitted by the probe. This is, disregarding scattering orother losses, equal to 0.9900P_(o). The value of α_(v) is derived fromthe difference (P_(o) -P)/P_(o). Assume now that at 300K the temperaturecoefficient of α_(v) is equal to 1.67 percent per K, that is, atemperature increase of 1K increases α_(v) by 1.67 percent. But onemeasures P, not α_(v), and when α_(v) changes by 1.67 percent P changesby only 0.017 percent, a change of less than 2 parts per 10,000. Such asmall change is difficult to measure accurately with ordinaryphotometric equipment.

Using the principles of this invention, one can measure α_(v) directly,and obtain an actual, directly measurable light intensity which doesvary by 1.67 percent per kelvin. To do this, one uses a probe asdescribed in the preceding paragraph, with the additional characteristicthat the absorbed light of wavelengths λ_(v) is converted by the probeinto luminescence light having wavelengths λ_(f) different from λ_(v).The intensity of this luminescence light, I_(f), will than be anapproximately linear function of α_(v) according to the relation

    I.sub.f =P.sub.o α.sub.v φ(λ.sub.v /hc) photons.sec.sup.-1

where

φ is the luminescence quantum efficiency of the probe material;

h is Planck's constant; and

c is the velocity of light in a vacuum.

If P_(o) is kept constant, then

    I.sub.f =Bα.sub.v photons,sec.sup.-1

where B is a constant.

It is important to be able to measure small attenuations accurately,especially with multiplexed or distributed sensors, in which theattenuation of the interrogating light per sensing point must be keptlow.

The above method is useful for increasing the accuracy of measurementsfrom any kind of optical attenuation, not just attenuation byabsorption. For example, microbending sensors work by forcing a fractionof the intensity of the interrogating light propagating along the coreof an optical fiber out of the core end into the fiber cladding.According to the teachings of this invention, one can incorporate aluminescent material or a Raman scattering material into the claddingand convert the light entering into the cladding under the action of themicrobending force into either luminescence light or Raman-shiftedlight, the intensity of which is directly proportional to the fraction αremoved from the core. One can then guide the converted radiation backalong the same fiber to the photodetection station.

Alternatively, one can carry the fraction α through a fiber cladding,without luminescence conversion but rendered separable from theinterrogating light being propagated through the core by the time domainseparation technique described in section 3.2 hereinafter. The spectralor temporal separation of the signals from the interrogating light, plussignal intensities which are orders of magnitude stronger thanRayleigh-backscatter signals, permit the multiplexing of a much greaternumber of sensing points on a single sensing fiber than is possible withthe prior art, for the same intensity of the interrogating light and thesame photodetector.

2.0 Specific Applications

2.1. Fiber Optic Temperature Sensors

The teachings discussed above can be used as a basis for constructingnovel sensors with greatly enhanced performance compared to the priorart. As discussed above in the section "Description of the Prior Art", anumber of fiber optic temperature sensors are based on opticaltransmission measurements of a temperature-dependent light absorption.In this section I describe improved sensors using vibronic materials andwavelength conversion of a temperature-dependent absorbed fraction ofthe intensity of the interrogating light, and present a theoreticalanalysis of some physical systems suitable for use in practicalembodiments. The analysis is deliberately oversimplified to emphasizethe aspects most relevant to the invention. It is not claimed that thequantitative relationships are followed rigurously in all practicalcases.

Preferred sensors use luminescent materials operated according to theprinciples described and illustrated with reference to FIG. 1. Theluminescent material has a ground electronic energy level whichcomprises vibronic levels 40, 41, 42, 43 and other levels which, for thesake of simplicity, are not shown. The lowest excited electronic levelcomprises vibrational sublevels 50, 51, and other vibrational sublevelsnot shown. The vertical arrowed line 60 represents an optical electronictransition produced by the absorbed excitation light, from level 42 tovibrational sublevel 50, which have fixed energies E_(v) and E_(s),respectively, relative to level 40. The length of line 60 corresponds tothe photon energy of the optical transition and, hence, to the specificwavelength λ_(v) of the excitation light. This wavelength obeys therelation λ_(v) =hc/(E_(s) -E_(v)), where h is Planck's constant and c isthe velocity of light in a vacuum. The wavelength λ_(v) can excite onlymolecules occupying vibronic level 42 and, to a smaller extent,molecules occupying slightly higher levels, the excitation of which isrepresented by the dotted vertical line 61. Luminescence emission occursfrom sublevel 50 to the vibronic levels of the group electronic level,said emission represented by lines 70, 71, 72 and 73. As shown in FIG.1, a considerable spectral portion of the emission occurs at photonenergies higher (and wavelengths shorter) than that of the excitationlight, and is commonly referred to as anti-Stokes luminescence.

In practice the vibronic material is often used as a solid solution,glassy or crystalline, in a transparent host material, said solidmaterial constituting the temperature probe. The concentration of thevibronic material and the dimension of the probe along the direction ofthe illuminating light are chosen so that the probe absorbs only afraction α_(v) of the nearly monochromatic excitation light within thetemperature range of operation, and transmits the rest. The absorbedfraction obeys the following relation:

    α.sub.v 32 1-10.sup.-ε c.sub.o d(N.sub.42 /N) (1)

where

ε is the molar decadic absorption coefficient of the molecules occupyingthe vibronic level 42;

c_(o) is the total molar concentration of the vibronic material;

d is the length of the sensor in the direction of the incidentexcitation light;

N₄₂ is the number of molecules of the vibronic material occupyingvibronic level 42; and

N is the total number of molecules of the vibronic material.

The ratio N₄₂ /N essentially follows the relation

    N.sub.52 /N=f.sup.-1 ·exp(-E.sub.v /kT)           (2)

where f is the so-called partition coefficient of the molecular system kis the Boltzmann constant, and T is the absolute temperature. Theexpression c_(o).f⁻¹ exp(-E_(v) /kT) is essentially the effective molarconcentration of the molecules of the vibronic material occupying thevibronic level 42, and the quantity 10⁻ εc_(o) d(N₄₂ /N) represents thefraction of the illuminating (interrogating) light which is transmittedby the probe, assuming no scattering and/or reflection losses, and equalto (1-α_(v)). Since E_(v) and k are both constant, the ratio E_(v) /kcan be designated by the single constant β, for a given fixed wavelengthλ_(v) of the interrogating light.

At optical densities no greater than 0.02, α_(v) is approximately givenby

    α.sub.v ≈2.3 εc.sub.o /df.sup.-1 exp(-E.sub.v /kT) (3)

At optical densities greater than 0.02 the relationship between α_(v)and the Boltzmann factor exp(-E_(v) /kT) becomes less linear, butequations (1) and (2) are still valid, and the method can be used athigh, low or intermediate optical densities.

The luminescence intensity I_(f) generated by the light absorbed by thesensor obeys the relation

    I.sub.f =P.sub.o (λ.sub.v /hc)α.sub.v φ photons·sec.sup.-1                               (4)

where

P_(o) is the radiant power, in watts, of the incident excitation light,and

φ is the luminescence quantum efficiency of the vibronic material.

Probes made from materials having high φ values can produce largesignal-to-noise ratios even with optical densities lower than 0.01,provided that the optical system has at least a moderately highcollection efficiency for the generated luminescence. Such efficiency iseasily obtainable with state-of-the-art fiber optic systems.

The sum of the light intensity absorbed and the light intensitytransmitted by a clear medium is constant. It follows, therefore, thatas the absorbed fraction α_(v) increases with an increase in temperatureaccording to equation (3), the intensity of the transmitted fractionmust decrease accordingly. Since, according to equation (4), theintensity of the luminescence light is proportional to α_(v), it followsthat the ratio of the intensity of the luminescence light to that of thetransmitted light increases with an increase in temperature, and theratio can be used as a temperature indicator. The ratio eliminates orminimizes any sources of error associated with fluctuations of theintensity of the illuminating light and fiber or connector losses.

The temperature coefficient of the luminescence intensity followsapproximately the relation

    (1/I.sub.o)(dI.sub.f /dT)=E.sub.v /kT.sup.2 =β/T.sup.2 (5)

where I_(o) is the luminescence intensity at a chosen referencetemperature. For example, a material with a value of E_(v) of 1200 cm⁻¹has a coefficient of about two percent per kelvin at a temperature of295K.

Equations (3) to (5) show that the method of the preceding paragraphsrequires only a temperature-dependent change in the optical absorptioncoefficient of the luminescent sensor material at wavelengthscorresponding to photon energies lower than the energy E_(s) of theexcited emissive level. This property is shared by virtually all solidand liquid luminescent materials. The method does not require anytemperature-dependent changes in the luminescence quantum efficiency,spectral distribution or decay time. Therefore, and in contrast to allother prior art methods, it can be implemented with most luminescentmaterials.

Experimental tests of equations (3) to (5) have been carried out withliquid solutions of three different dyes dissolved in dimethyl sulfoxide(DMSO). Two of the dyes, dye I and dye II are represented by thechemical structures ##STR1## Dye I is the sulfonate derivative ofHostasol Red GG (American Hoechst Corp.). Dye II has been described inU.S. Pat. No. 4,005,111 by Mach et. al. The third dye was the well knownRhodamine 6G (R6G). The dyes were dissolved in DMSO at concentrations ofthe order of 10⁻⁴ Molar and excited with light from a He-Ne laser (λ=633nm) in a square cuvette. The fluorescence intensity was monitored at thewavelength of 612 nm, shorter than the wavelength of the excitationlight. The experimentally measured fluorescence intensities I_(f) weremeasured as a function of the absolute temperature T. Plots of I_(f)v·T⁻¹ are shown in FIG. 2 for the three dyes. The behavior predicted byequations (3) and (5) was confirmed. The slopes of the lines drawnthrough the experimental points give E_(v) values of 1380, 1355 and 1890cm⁻¹ for dyes I, II and R6G, respectively. When these values are addedto the excitation photon energy of 15803 cm⁻¹, one obtains E_(s) valuesof 1.72×10⁴ cm⁻¹ for dyes I and II, and 1.77×10⁴ cm⁻¹ for R6G. Thesevalues are in good agreement with the E_(s) values determined from thefluorescence spectra of these dyes.

The validity of equations (3) and (5) is independent of whether thesensor is a liquid or a solid. Although the laboratory reduction topractice was done with liquid solutions of these dyes, solid plasticsolutions are preferred for practical measurements. Examples of suitableplastics are polystyrene, polymethyl methacrylate, polyurethanes andsilicones.

An important advantage of this method for measuring temperature is thatthe value of E_(v), which determines the optimum temperature range ofoperation, can be chosen and varied at will over a continuum of valuessimply by choosing, for any given vibronic material, the photon energyof the excitation light relative to the energy E_(s) of the excitedemissive level (sublevel 50 in FIG. 1). Thus, a single sensor can beused for measuring temperature over a wide range, from cryogenictemperatures up to the highest temperatures which the sensor canwithstand without deterioration or severe thermal quenching of itsluminescence. An additional advantage derives from the fact that thereare many luminescent vibronic materials having absorption andluminescence spectra over a wide spectral region from the ultraviolet tothe near infrared. One can choose, therefore, the wavelength region mostsuitable to one's needs. For instance, if it is required to transmit theoptical signal over long distances via an optical fiber, one couldchoose a material with absorption and luminescence bands at wavelengthslonger than 600 nanometers (nm).

Examples of vibronic materials suitable for measuring temperatureaccording to this invention are plastic-soluble fluorescent dyes, likedyes II and R6G used for obtaining the data of FIG. 2. Other preferreddyes are those with excitation and fluorescence bands within thespectral region in the red and near infrared for which efficient andinexpensive light-emitting diodes (LED's) and laser diodes (LD's) arecommercially available. These include dyes of the violanthrone family,including but not limited to the dye having the Color Index designationof Vat Green 1, having the chemical structure ##STR2## The solubility ofthese dyes in plastics can be increased by attaching to themsolubilizing substituent groups like long chain or branched chain alkylradicals. In dye Vat Green 1, for instance, the methyl (Me) radicalsshown can be replaced by tert-butyl or longer chain radicals bytechniques well-known to chemists.

Other vibronic materials suitable for measuring temperature according tothe teachings of this invention include inorganic crystals doped withchromium(III) or other transition metal ions like nickel(II), cobalt(II)or vanadium(II).

Another class of vibronic materials, also suitable for use astemperature sensors according to this invention, are described withreference to FIG. 3, which shows a simplified molecular energy leveldiagram somewhat similar to that of FIG. 1. What is different is thatthe emissive energy level is different from the level which is directlyexcited by the excitation light. Referring to FIG. 3, excitation ofmolecules occupying a vibronic level proceeds in the same manner asshown in FIG. 1 for the materials described hereinbefore. Levels 40A,41A, 42A, 43A, 50A and 51A are similar to levels 40, 41, 42, 43, 50 and51, respectively. The same digits in both figures indicate thesimilarity of the excitation processes, and the A's were added to thelevels of FIG. 3 to indicate that these levels belong to a differentclass of vibronic materials. The difference is that in this case theoptically excited level 50A transfers at least a major part of itsexcitation energy, via a radiationless decay represented by the wavyline 55, to the lower level 56, of the same or of a different molecularspecies. Luminescence occurs from level 56 to a lower level 57 or to anyother lower-lying levels which may or may not include any of the levels40A, 41A, 42A or 43A. Examples of this class of vibronic materialsinclude virtually all Triplet→Singlet phosphorescent dyes, luminescentchelates of terbium(III) and europium(III), and some solid solutions ofinorganic vibronic materials co-doped with other luminescent centers.Examples of the latter are crystalline or glassy materials co-doped withchromium(III) and neodymium(III), in which chromium(III) ions absorb theexcitation light and transfer their excitation energy to theneodymium(III) ions, causing them to luminesce.

Some sensor materials can behave either according to the model of FIG. 1or the model of FIG. 3, depending on the temperature range in which theyoperate. For example emerald, a chromium(III)-doped beryl, behavesaccording to the model of FIG. 1 at temperatures higher than ambient,and according to the model of FIG. 3 at cryogenic temperatures.

Two other classes of materials suitable for the practice of thisinvention are: (a) luminescent lanthanide ions dissolved at relativelyhigh concentrations in crystalline or glassy hosts, and having at leastone electronic energy level which can be thermally populated to ameasurable extent at the temperatures being measured, and (b)luminescent semiconductors with a temperature-dependent absorption edgewavelength. Both cases of materials are characterized by an absorptioncoefficient which, within a relatively narrow spectral region, increasesapproximately exponentially with increasing temperature in essentiallythe same manner as with the vibronic materials described hereinbefore.Therefore, they are used in essentially the same manner and with thesame methods as discussed above.

In the above-described embodiments the luminescent centers had atemperature-dependent absorption coefficient to light of photon energylower than that of the electronic level being excited. These centers aredesignated herein as A centers. One preferred procedure for referencingthe signals from these centers is to incorporate in the same mediumother luminescent centers, designated herein as B centers, theabsorption coefficient and luminescence efficiency of which areapproximately independent of temperature within the temperature rangebeing measured, when excited with light of the same wavelength λ_(v) asthe A centers, and the luminescence of which is emitted in a spectralregion different from that of the A centers. If one divides theluminescence intensity emitted by the A centers by the luminescenceintensity emitted by the B centers, the ratio so obtained is a uniquefunction of temperature. This referencing technique is discussed furtherhereinafter in connection with continuous, distributed temperaturesensors using a single long, unbroken temperature-sensing optical fiber.

When using a non-crystalline sensor material, it is convenient to makeit the core of an optical fiber segment, with an appropriate claddingaround the core, for easy connection to an optical fiber wavelength.

If the A centers and the B centers have different luminescence decaytimes, then one can determine the relative intensities of theluminescence emitted from the two kinds of centers by measuring thetotal luminescence decay time. For example, an yttrium aluminum garnet(YAG) crystal co-doped with ND³⁺ and Yb³⁺ ions can be excited with lightof wavelengths near 946 nm or any other wavelength at which theabsorption by Nd³⁺ is strongly temperature-dependent but the absorptionby Yb³⁺ is only minimally affected by temperature. The luminescence fromNd³⁺ peaks at about 1064 nm, and that from Yb³⁺ peaks at about 975 nm.The relative intensities of both luminescences can be measured by usingspectrally selective optical filters and measuring separately theluminescence intensities at 975 and 1064 nm. A simpler way, which doesnot require optical filters, is to pulse the interrogating light andmeasure the decay time of the total luminescence from both Nd³⁺ andYb³⁺. The luminescence decay time of Yb.sup. 3+ is several times longerthan that of Nd³⁺. Thus, as the temperature increases, the luminescenceintensity from Nd³⁺ increases relative to that from Yb³⁺, and the decaytime of the total luminescence decreases as a function of said relativeintensities. Instead of a crystalline matrix, the Nd³⁺ and Yb³⁺ ions maybe dissolved in a glass matrix. Except for the use of a glassy medium,the probe is operated in the same manner as described in the precedingparagraph.

An optical fiber temperature probe based on the measurement of atemperature-dependent light absorption measurement has an advantage overother optical probes in that it measures the average temperature overthe length of the fiber probe. Thus, if one wishes to measure theaverage temperature over, for example, a pipe having differenttemperatures along its length, a long fiber temperature probe disposedover the whole length of the pipe will measure its average temperature.A preferred embodiment uses a luminescent probe with atemperature-dependent light absorption, the luminescence intensity ofwhich follows approximately equation (4).

Fiber Optic Temperature Sensors Usable from the Cryogenic Region up toOver 850° C.

The teachings of section 2.1 can be used for constructing a fiber opticcryogenic thermometer usable from the liquid helium region up to over850° C., using a single probe. One preferred embodiment uses as a probea single crystal fiber of the well-known material Nd:YAG. The upperlimit of operation of the probe is determined by the temperature beyondwhich substantially quenching of the luminescence occurs, and this ishigher than 850° C. for Nd:YAG. The principles of operation of the probecan be understood with reference to the physical model of FIG. 1, andequations (3)-(5). As follows from the discussion in section 2.1, themagnitude of the energy gap E_(v), and hence of the Boltzmann factorexp(-E_(v) /kT) for any temperature range, can be chosen simply bycontrolling the wavelength λ_(v) of the excitation light. Eachtemperature range of operation has an optimum range of E_(v) magnitudes.The requirements are usually a moderate absorption coefficient to theexcitation light, typically corresponding to a value of α between 0.02and 0.20, combined with a temperature coefficient of α preferablygreater than 0.01 per kelvin. The optimum magnitudes of E_(v) thusincrease with increasing temperature. Suppose that one desires atemperature coefficient of about 0.015 or greater per kelvin at ambientor lower temperatures. According to equation (5) the coefficient isequal to (E_(v) /kT²). From this one obtains magnitudes for E_(v) ofabout 910 cm⁻¹ at the ambient temperature of 295K (22° C.) and 130 cm⁻¹at 112K, the boiling point of liquefied natural gas (LNG).

At temperatures higher than 600K, practical temperature coefficients ofα are lower. For example, at 723K (450° C.) a temperature coefficient of0.015 would require an E_(v) of 7,841 cm⁻¹, corresponding to a Boltzmannfactor of about 1.7×10⁻⁷ and, hence, to a negligible value of α.Temperature coefficients of a few tenths of 1 percent are practical,though, which are compatible with temperature accuracies of about 1percent.

The measurement of cryogenic temperatures.

Conditions are more favorable at cryogenic temperatures. For example, acomfortable Boltzmann factor of 0.05 at 10K, corresponding to amagnitude of about 20 cm⁻¹ gives a temperature coefficient of α of about30 percent per Kelvin, which should make it possible to measuretemperature changes smaller than 0.01K.

One important need in cryogenic thermometry, especially near 4.2K, isthe measurement of temperatures in the presence of high magnetic fieldsgenerated by superconducting magnets. Electrical thermometers areusually unsuitable for this purpose, as their performance is affected bythe magnetic field. Very high magnetic fields, of the order of 1 teslaor greater, can also affect optical probes like Nd:YAG, by changing theposition of either the E_(s) or that of the E_(v) level. But the effectof these changes, if any, can be cancelled out by carrying out themeasurements at two slightly different wavelengths λ_(v1) and λ_(v2) ofthe interrogating light corresponding to E_(v) values of E_(v1) andE_(v2) respectively, for instance by tuning the laser wavelength of aninexpensive AlGaAs diode laser. Any specific wavelength differenceΔλ_(v) corresponds to a specific difference ΔE_(v) independent of theeffect of the magnetic field on the individual magnitudes of E_(v1)and/or E_(v2). The temperature T will then be determined from therelation

    T=ΔE.sub.v ;k·ln(I.sub.2 /I.sub.1)          (6)

where I₁ and I₂ are the luminescence intensities generated by theinterrogating light wavelengths λ₁ and λ₂, respectively.

2.2. A Distributed Temperature Sensing System.

The advantages of the temperature sensing of this invention, discussedin the preceding section, can best be appreciated by its adaptability tomeasure temperatures in a multiplicity of locations simultaneously, withthe probes at each location being part of a series array of sensorsalong a fiber-guided light path. Such an application requires accuratemeasurements of small changes in the intensity of the light propagatingalong the array, as the multiplexing of many light-absorbing probesalong a single light path requires that each probe absorb only arelatively small fraction of the intensity of the interrogating light.One suitable embodiment uses individual probes, conveniently in the formof optical fibers, spliced at different locations into a long opticalfiber guide. A logical extension of this concept is to dope the wholecontinuous length of a long fiber with the light-absorbing luminescenceconverter. Such an embodiment is a preferred one when the number ofsites to be monitored is large, or when checking for hot spots along acontinuous path, for example along the windings of a large electricaltransformer. As described above, one can measure small opticalabsorptions directly by converting the fraction of the intensity of thelight which is absorbed into luminescence or Raman emission.

A general optical time-domain reflectometry method and apparatus forsensing magnitudes (values) of a physical parameter at different sensingpoints along an optical fiber pathway using wavelength conversiontechniques was described by applicant in the parent application Ser. No.711,062 (now U.S. Pat. No. 5,004,913), in the sections entitled "I. ALong, Continuous Optical Fiber Sensitive to Perturbations Along ItsWhole Length" and "J. The Use of Luminescence in Time DivisionMultiplexed Systems" (U.S. Pat. No. 5,004,913, col. 26 line 62 to col.29 line 48), the teachings of which are incorporated herein. The "TimeDivision Multiplexed Systems", illustrated in FIGS. 21 and 22 of saidpatent are generally known to those of ordinary skill as opticaltime-domain reflectometry (OTDR) devices. Their application to themeasurement of distributed temperature is described here.

If one uses as a light-absorbing material a fluorescent dye with afluorescence decay time of the order of nanoseconds (characteristic ofmost fluorescent dyes), then one can use OTDR techniques for convertingany light absorbed at any point along the fiber length into afluorescence light intensity free of the baseline background of theinterrogating light. If, for example, a resolvable fiber segment lengthabsorbs one percent of the interrogating light intensity at 300K, andthe temperature coefficient of the absorption is 1.2 percent per kelvin,the fluorescence intensity will change by 1.2% per kelvin, while theintensity of the transmitted light, as measured by Rayleighbackscattering, will change by only about one part per 10,000.

In a practical embodiment one must be able to measure both thetemperature and the location of any hot or cold spot. Locationinformation can be obtained by standard OTDR techniques, from the timeof arrival of the fluorescence light pulses at the electro-optic unit,relative to the time of launching of the interrogating light pulse intothe fiber. The following relation holds:

    d=0.5 tc/n

where

d is the fiber distance from the electrooptial unit (where theinterrogating light pulses were launched);

t is the time of arrival of the fluorescence light pulses;

c is the velocity of light in a vacuum; and

n is the index of refraction of the glass of the fiber core.

The temperature of the fiber at any distance d from the electroopticunit is determined from the intensity of the fluorescence pulse.

The spatial resolution of the measuring system is determined by therisetime and duration of the interrogating light pulses, and by thefluorescence decay time of the fluorescent material. With fluorescencepulses with a duration of 10 nanoseconds one can obtain spatialresolutions of about 1 meter or better.

Two alternate embodiments are described below:

In the first embodiment, applicable when the fiber length does notexceed about 100 meters, the fiber consists of a plastic core having thefluorescent dye dissolved therein, and plastic cladding with an index ofrefraction lower than that of the core, but with a temperaturecoefficient of said index of refraction similar to that of the core.Only minute concentrations of the dye are needed, so there is a largenumber of efficient fluorescent dyes available, including dyes II andR6G subject of FIG. 2. Since virtually all plastic-soluble fluorescentdyes can be used for the practice of the invention, it is convenient touse those which can be interrogated with light from inexpensive LED's orlaser diodes, and which have good stability. Dyes of the violanthrone(dibenzanthrone) family, including Vat Green 1, have thesecharacteristics.

For accurate measurements it is necessary to provide a reference signal.The intensity of the Rayleigh-backscattered light can be used for thispurpose. The ratio of the intensity of the dye fluorescence to that ofthe Rayleigh-backscattered light provides a reliable indication of thetemperature at each resolvable fiber length.

If longer lengths of the temperature-sensing fiber are needed, thesecond embodiment is preferable. In this case one uses an optical fiberhaving a core made of low-attenuation glass, for instance pure silica,and the fluorescent dye is dissolved in a transparent plastic cladding.The index of refraction of the plastic cladding is lower than that ofthe core, and excitation of the dye fluorescence is accomplished byevanescent wave coupling. Since the light paths of both theinterrogating light and the fluorescence light are throughlow-attenuation glass, light signals can be collected over much longerdistances than possible from an all-plastic fiber. Signal referencingcan be obtained by co-dissolving in the plastic cladding a secondfluorescent dye the absorption coefficient and fluorescence efficiencyof which are essentially invariant over the temperature range to bemeasured. The relative fluorescence intensities from the two dyesuniquely define the temperature of the fiber at the measured location.The second fluorescent dye (the reference dye) can be chosen from anyfluorescent dye soluble in the plastic cladding and having a fluorescentlevel with an energy no higher than the photon energy of theinterrogating light.

A suitable embodiment of a device (essentially an optical time-domainreflectometer with an added fluorescence channel) for performingdistributed temperature measurements with either of the above fiberembodiments is shown schematically in FIG. 4. The light source 10 isdriven to produce interrogating light pulses with a duration of bout 10to 30 nanoseconds and a wavelength λ_(v). These light pulses areinjected into the fiber segment 11 and, through the optical fibercoupler 12, into the temperature-sensing fiber 13. At any point alongthe fiber, each interrogating light pulse produces a fluorescence lightpulse with an intensity determined by the temperature at that point, anda reference pulse at a different wavelength from thattemperature-dependent pulse. In the case of the plastic fiber thereference pulse is the intensity of the Rayleigh-backscattered light. Inthe case of the fiber with the glass core and fluorescent plasticcladding, the reference pulse is the fluorescence light pulse producedby the co-dissolved second fluorescent dye, this dye having atemperature-independent optical absorption coefficient at the wavelengthof the interrogating light. Both the temperature-dependent light pulsesand the reference pulses are transmitted by the optical fiber to theelectrooptial unit and, through optical coupler 12 and fiber segments 14and 15, to photodetectors 16 and 17. The photodetectors are madespectrally selective, one of the temperature-dependant optical pulses,an the other to the reference pulses, by means of narrow ban-passoptical filters applied to their windows. The time of arrival at thephotodetectors of the optical pulses generated at any point along thefiber, relative to the time of generation of the interrogating lightpulse at the light source, defines the location along the fiber wherethe optical pulses are generated. The ratio of the photoelectric signalfrom the fluorescence generated by the temperature-dependent absorptionto the reference signal is an indicator of the temperature at that pointalong the fiber. When such ratio is recorded as a function of the fiberdistance from the fiber tip into which the interrogating light pulsesare launched the above-background temperatures appear as peaks, and thebelow-background temperatures appear as depressions in graph 4A.

2.3. The Measurement of Temperature Distributions by anti-Stokes RamanScattering.

The preceding sections described how one can use a material formeasuring temperature, by causing it to convert a temperature-dependentfraction α_(v) of the intensity of illuminating light of pre-selectedwavelength λ_(v) into light emitted at a wavelength different fromλ_(v). The above cited anti-Stokes Raman technique by Dakin et. al.,Electron. Lett. 21, 569 (1985), can be regarded as a special case ofthis general technique. It also uses a temperature-dependent excitationof molecules occupying thermally populated vibronic levels, except thatthe optical excitation is to a virtual energy level rather than to anactual one. The process is illustrated by FIG. 5. As in the case ofluminescent vibronic materials, the occupancy number N₄₂ is determinedby the Boltzmann factor exp(-E_(v) /kT). The above equations (2) and (5)are also approximately obeyed. But the fraction α_(v) of theinterrogating light which can be emitted as Raman-scattered light isusually orders of magnitude lower.

3.0 Distributed Sensing of Physical Variables By Light RedistributionBetween Two Light-Guiding Regions of an Optical Fiber Probe

If an optical fiber has two light-guiding regions, and a physicalvariable can cause measurable, variable redistribution of interrogatinglight between these regions, then one can use such a fiber as a sensingprobe for the physical variable. If, additionally, such lightredistribution at any sensing point along the fiber does not affectsignificantly the relative light distribution at other sensing points,then the fiber probe can be used as a distributed sensor. It is shownbelow how one can measure distributed temperatures and other physicalvariables using this principle.

3.1. An Alternate Method for Measuring Distributed Temperatures with aSingle Unbroken Optical Fiber Probe

Another method for measuring distributed temperatures according to thisinvention uses a temperature-dependent light distribution between twolight-guiding regions of an optical fiber. One of the preferredembodiments uses a fiber shown schematically in FIG. 6. It comprises aglass core A (the first light-guiding region) with an index ofrefraction n₁, a first cladding B (the second light-guiding region),having a temperature-dependent index of refraction n₂ lower than n₁, anda second cladding C around the first cladding, having an index ofrefraction n₃ lower than n₂. The launch end of the fiber is aluminizedon the face of the two claddings, so that the interrogating light can belaunched only through core A. The interrogating light is launched asrecurrent train of short pulses with a duration of the order of a fewnanoseconds (ns) or shorter, depending on the spatial resolution desired(approximately 10 ns per meter), over an acceptance angle θ formeridional rays necessary to fill the numerical aperture (NA)₂ definedas

    (NA).sub.2 =(n.sub.2.sup.2 -n.sub.3.sup.2).sup.1/2

In other words, the interrogating light fills both light-guiding regionsA and B. The value of n₂ decreases in a known manner with an increase intemperature, at a much greater rate than the decrease in the value ofn₁, and the intensity distribution of the interrogating light pulsesbetween regions A and B will be determined by the relative values of thesquares of (NA)₂ and the numerical aperture (NA)₁ of core A, defined as

    (NA).sub.1 =(n.sub.1.sup.2 -n.sub.2.sup.2).sup.1/2

The value of (NA)₂ can be kept approximately constant over the workingtemperature range by making cladding C out of a material such that itsindex of refraction n₃ has essentially the same temperature coefficientas the index of refraction n₂ of cladding B.

Now, the intensity of the light Rayleigh-backscattered from the core Aat any resolvable segment of the fiber, corrected for the intrinsiclight attenuation of the fiber per unit length, will e a known functionof the temperature of the segment. In contrast to prior art methods oftemperature measurement using a temperature-dependent index ofrefraction, cross-talk between different sensing points is minimized byvirtue of the fact that light rays deflected out of core A through atemperature change are not `thrown away` as in the prior art, butcaptured and returned to the region comprising core A and first claddingB, thus restoring a temperature-dependent light distribution at everysegment of the fiber.

Since the cladding faces of the fiber are aluminized (or otherwise madeopaque), and the diameter of cladding B can be made much larger thanthat of core A, the intensity of the Rayleigh-backscattered lightcollected at the core launch end from any resolvable segment of thefiber will be proportional to the intensity of the interrogating lightpropagating within the core at that segment, determined by the value of(NA)₁ and, hence, by the temperature-dependent value of n₂. Anycontribution from cladding B to the collected Rayleigh-backscatteredlight can be further minimized by using a small collection angle φconsistent with the needed signal intensity.

The sensitivity and performance of the proposed distributed sensor maybe anticipated from the following assumed conditions:

The sensing fiber comprises a core A made of silica glass with an indexof refraction n₁ of 1.45800, an elastomeric silicone cladding B with anindex of refraction n₂ of 1.45030 at the temperature of 300K, and asecond silicone cladding C with an index of refraction n₃ at 300K of1.141900. Both n₂ and n₃ have a temperature coefficient of 2×10⁻⁴ perkelvin. Then a change of 1 kelvin from 300K to 301K will increase thevalue of (NA)₁ ², and hence of the intensity of theRayleigh-backscattered light, by 2.6 percent, a rather large changecompared to prior art fiberoptic temperature-sensing systems.

The spatial resolution of the system depends on the risetime and/orduration of the interrogating light pulses. Using an optical time domainreflectometer (OTDR) and light pulses with a duration of 100 picoseconds(ps), for example, one could obtain a spatial resolution of the order of10 cm, limited by time dispersion effects in the fiber.

Another embodiment of these sensors uses fibers wherein both claddings,as well as the core, are made of glass, with the first cladding havingan index of refraction with a temperature coefficient different fromthat of the core. While glass has lower temperature coefficients for itsindex of refraction than plastics, the change may be sufficient for anumber of applications, especially at high temperatures at which aplastic cladding would degrade.

In another embodiment, one uses an optical fiber as described above andhaving the additional property that the light scattering coefficient ofthe central core is knowingly and substantially different from that ofthe first plastic cladding. In that case the intensity of thebackscattered light pulses generated and captured within the regioncomprising both the core and the first cladding, and back-directed tothe fiber launch end, is measured by ordinary OTDR techniques. Theintensity of the backscatter pulses generated at each sensing point,relative to the intensity of the forward interrogating light pulses, isthen an indicator of the fiber temperature at that point.

For example, assume a fiber wherein the plastic claddings have atemperature coefficient of n₂ of 3×10⁻⁴.deg⁻¹, and a light scatteringcoefficient 5 times greater than that of the glass core. Assume that ata temperature T₀ =300K, the index of refraction n₁ of the glass core isequal to 1.4600, while that of the first cladding, n₂, is equal to1.4400. Then, (NA)₁₂ =0.2408. Now, if the temperature is increased by50° C., n₂ will decrease to 1.4250, the magnitude of (NA)₁₂ willincrease to 0.3178, and the intensity of the interrogating lightpropagating within the first cladding will decrease to 57.4% of itsoriginal value, while the light propagating along the central core willincrease by about 74 percent. If at 300K the intensity of theinterrogating light was distributed equally between the glass core andthe first plastic cladding, then the total backscatter intensity at 350Kwill be decreased to about 77% of its intensity at 300K.

In yet another embodiment, suitable for measuring temperatures lowerthan ambient, the fiber probe has a glass core and two concentricplastic claddings as described above, except that the second cladding Chas dissolved therein a relatively low concentration of a fluorescentdye. The interrogating light is launched into the glass core only, so asto fill its numerical aperture (NA)₁. A temperature decrease at anypoint along the fiber will increase the index of refraction n₂ of thefirst cladding B. This will decrease the value of (NA)₁, causing afraction of the intensity of the interrogating light to be deflectedfrom the core to the first cladding B and the boundary between the firstand second cladding, where it generates a fluorescent signal theintensity of which relative to that of the interrogating light is aunique function of temperature. The dye concentration in cladding C ispre-selected so that a small fraction only of the intensity of theinterrogating light deflected out of the core is absorbed by thefluorescent cladding. Thus, as the interrogating light pulses continuetheir propagation along the fiber beyond the cold spot, most of theintensity of the deflected light will re-enter the fiber core and willbe available for interrogating the rest of the fiber.

A temperature sensing fiber probe as described in the precedingparagraph is particularly useful for detecting leaks of cryogenicfluids, for example liquefied hydrocarbons in petrochemicalinstallations, or rocket fluids during rocket launchings.

3.2. The Measurement of Distributed Forces with a Single Unbroken FiberProbe.

Mechanical forces acting on an optical fiber, especially microbendingforces, usually cause an attenuation of light being transmitted throughthe fiber core, by deflecting a fraction of the intensity of this lightout of the core and into the fiber cladding. If such forces are actingon a plurality of points on a long fiber, or at a single but unknownlocation on it, they can usually be measured by Optical Time DomainReflectometry (the abbreviation OTDR is used herein both for the methodand for the device used for implementing it). The method consists oflaunching interrogating light pulses with a duration typically of theorder of nanoseconds (ns) into the fiber core, and measuring theintensity of the Rayleigh-backscattered light pulses as a function offiber distance from the fiber tip at which the interrogating light pulsewere launched. Any force on the fiber which causes an attenuation of theintensity of the interrogating light pulses (for instance a microbendingforce) is revealed as a discontinuity in the backscatter intensityversus distance decay curve. The time of arrival of theRayleigh-backscattered pulses, relative to the time of launching of theinterrogating light pulses, defines the location along the fiber wherethe force is acting, and the intensity of the backscattered pulsesindicates the magnitude of the force.

A serious shortcoming of this method is that it `throws away` thedeflected light to be measured, and instead estimates its magnitudeindirectly from a difference between two usually much larger, but stillweak and noisy backscatter signals. Rayleigh scattering is aninefficient process, producing intensities at the photo-detector whichare typically 50 or more dB down from the forward interrogating lightintensity at any 1 meter length of fiber. These indirect measurementsare, then, plagued by a relatively large amount of baseline noise.

The teachings of this invention provide a method and associated devicesfor the measurement of force distributions on optical fibers, as well astemperature distributions. The method and devices of this invention arecapable of producing, in the measurement of forces, signals stronger byorders of magnitude than those obtainable from Rayleigh-backscattermeasurements, for the same extent of optical attenuation and with thesame intensity of the interrogating light. The method consists ofconverting the fraction of the intensity of the interrogating lightwhich has been deflected into the fiber cladding into a signal separablefrom the interrogating light transmitted through the fiber core. Thisseparation can be achieved either in the optical wavelength domain, byluminescence or Raman conversion, or in the time domain, as illustratedin the following embodiments.

1ST EMBODIMENT

This embodiment uses a fiber shown schematically in FIG. 7, consistingof a glass core 1 with an index of refraction n₁ and a cladding 2 withan index of refraction n₂ lower than n₁. Around cladding 2 there is asecond cladding, concentric layer 3, containing fluorescent centersabsent in either the core or in cladding 2 and having a luminescencedecay time of the order of nanoseconds, this cladding having an index ofrefraction n₃ not lower than n₂ and a thickness of a few micrometers.Around cladding 3 there is a third cladding 4 with an index ofrefraction n₄ substantially lower than n₃. Interrogating light pulseswith a duration of the order of nanoseconds, having wavelengths withinthe absorption band of the fluorescent centers, are launched into thecore 1 of the fiber, where they propagate along the fiber axis with asmall attenuation caused by residual optical absorption with acoefficient ε and, usually to a larger extent, by Rayleigh lightscattering with a coefficient β. At a point A along the fiber, thelaunched interrogating light pulses have an intensity P_(o). After thispoint, and for a length L corresponding to the time resolution of theOTDR, the light intensity is attenuated by a fraction a_(t). The powerP_(s) of the Rayleigh-backscattered light pulses generated within thefiber segment of length L within the fiber numerical aperture (NA)_(s)and sent to the photodetection station is given approximately by

    (P.sub.s /P.sub.o)≈1/2a.sub.t [β/(β+ε)]·[(NA).sub.5.sup.2 /4n.sub.1.sup.2 ]f.sub.t watts                                            (6)

where the terms within the second square brackets define the fraction ofthe power of the scattered light captured within the solid acceptanceangle of the fiber core, and f_(t) is the fraction of the power of thiscaptured light which is transmitted by the fiber along the optical pathto the photodetector. The numerical aperture (NA)_(s) is given by

    (NA).sub.s =(n.sub.1.sup.2 -n.sub.2.sup.2).sup.1/2         (7)

An external force operating on this fiber segment and capable ofproducing a localized strain or any other microbending force willdeflect a fraction α_(s) of the power P_(o) of the interrogating lightpropagating along the fiber core at that point into cladding 2, dueeither to fiber microbending or to the increase of the index ofrefraction n₂ of the glass cladding relative to that of the core, thevalue of α_(s) being determined by the magnitude of the force. ThenP_(s) will be decreased by this fraction, and the change ΔP_(s) (thesignal) will be given by the relation

    ΔP.sub.s =α.sub.s P.sub.s watts

It should be noted that the optical processes described above involveonly core 1 and cladding 2 and are, therefore, unaffected by laser 3 orcladding 4.

Let us now look at what happens to the light forced out of the glasscore 1 and into cladding 2. For this purpose, we must treat the fiber ashaving a second numerical aperture (NA)_(f), for the generatedfluorescence, defined as

    (NA).sub.f =(n.sub.2.sup.2 -n.sub.4.sup.2).sup.1/2         (8)

Light entering cladding 2 at angles within (NA)_(f) will be trapped andpropagated along the fiber axis by total internal reflection fromcladding 4 but, after a number of reflections determined by theabsorption coefficient and concentration of the fluorescent centers inlayer 3, will be absorbed by these centers and converted intofluorescence with a quantum efficiency φ. The fluorescence will usuallybe emitted at longer wavelengths than those of the absorbedinterrogating light. The fraction P_(f) of the power of thisfluorescence light which reaches the photodetector is givenapproximately by

    P.sub.f =1/2α.sub.s P.sub.o φ[(NA).sub.f.sup.2 /4n.sub.2.sup.2 ]f.sub.t '(λ.sub.s /λ.sub.f) watts          (9)

where

f_(t) 'is the fraction of the fluorescence power generated within(NA)_(f) which is transmitted by the fiber along the optical path to thephotodetector;

λ_(s) is the wavelength of the interrogating light; and

λ_(f) is the mean wavelength of the fluorescence light.

Let us now assume some typical values for some of the parameters inequations (6) and (9), for a fiber segment one meter long:

    ______________________________________                                        a.sub.t =   2.3 × 10.sup.-3                                                                       (NA).sub.f =                                                                           0.40                                       [β(β + ε)] =                                                            0.80          n.sub.1 =                                                                              1.500                                      φ =     0.70          n.sub.2 =                                                                              1.487                                      f.sub.t.sup.'f.sub.t =                                                                    0.70          (λ.sub.s /λ.sub.f)                                                       0.90                                       (NA).sub.s =                                                                              0.20                                                              ______________________________________                                    

Stress or microbending forces which attenuate the intensity of theinterrogating light by not more than a few percent will deflect intocladding 2 only optical rays within the numerical aperture (NA)_(f) ofthe fiber, especially in view of the high value of (NA)_(f) relative to(NA)_(s). Thus, equation (9) applies, and one can determine from theabove values of the relevant parameters that

    P.sub.f /ΔP.sub.s ≈975

Fluorescence conversion of light forced out of the core of an opticalfiber by stress or other forces can, thus, produce signals orders ofmagnitude stronger than those obtained from the decrease of theRayleigh-backscattered light produced by the same forces. As aconsequence of this, distributed sensing systems can be designed so thateach sensing point attenuates a small fraction only of the intensity ofthe interrogating light propagating along the fiber at that point, thusallowing a given intensity of the interrogating light launched into thesensing fiber to interrogate, and produce strong signals from, a muchgreater number of sensing points than would be practical with a systembased on Rayleigh-backscattered signals.

The above-described fluorescent claddings can be applied to maximizediagnostic information from various types of fibers, including multimodestep index, graded index and single mode fibers. In the latter case, ofcourse, only the interrogating light beam is single mode. Cladding 3 canbe, for example, a clear plastic doped with an organic fluorescent dye.

SECOND EMBODIMENT

In the second embodiment the light forced out of the fiber core by astress or microbending force is converted into Raman-scattered light ata wavelength different from λ_(s). This requires a different fiber fromthat used in the first embodiment. The fiber consists of the same orsimilar glass core 1 as in FIG. 6. Around this core is a glass claddinghaving a relatively high Raman-scattering coefficient, for example asilica glass doped with a high concentration of phosphorous pentoxide,P₂ O₅, as Raman-active material. It is important that the fiber core donot contain the Raman-active material present in the cladding. Also, thecladding volume per unit length should preferably be greater than thecore volume. Light forced out of the core and into the cladding isconverted into Raman-scattered light. Generally, the Raman conversionprocess is less efficient than fluorescence conversion, so that thesignal strengths are smaller than possible with fluorescence conversion,at least for fiber lengths of the order of a few meters.

THIRD EMBODIMENT

The third embodiment is advantageous when the fluorescent material is anorganic dye, and the dye can be dissolved in a transparent plasticcladding having an index of refraction substantially lower than n₂, theindex of the glass cladding of FIG. 7. This will become apparent fromthe following discussion.

Even the clearest plastics attenuate light by at least an order ofmagnitude more than the best fiber-grade glass. It is because of thisfact that that the fluorescent layer 3 was specified to be only a fewmicrometers thick. If the diameter of cladding 2 is, for instance, 125micrometers (a common value), then only a small fraction of the opticalpath of the fluorescence light is through a high loss material. Butthere is a better and simpler way to achieve this objective, asdescribed below.

Fluorescence Conversion by Evanescent Wave Coupling.

The force-sensing fiber of this embodiment is illustrated in FIG. 8. Itcomprises the same core and the same cladding 2 of FIG. 7. Aroundcladding 2 there is a second cladding, coating 4', made of a transparentplastic and having dissolved therein a fluorescent dye characterized bya high quantum efficiency, an absorption band in a spectral regioncomprising the wavelength of the interrogating light, λ_(s), and afluorescence decay time of the order of 10⁻⁸ seconds or shorter. Theplastic has an index of refraction n_(p) low enough to produce anumerical aperture (NA)_(fp) higher than 0.3, and preferably higher than0.4, (NA)_(fp) being defined by the relation

    (NA).sub.fp =(n.sub.2.sup.2 -n.sub.p.sup.2).sup.1/2        (10)

Light entering cladding 2 from core 1 at angles greater than thecritical angle θ for total internal reflection from coating 4' willenter this cladding at least to a depth d_(p) known as the thickness ofthe evanescent layer, and defined as the depth over which the electricalfield amplitude of the light waves decays to the value 1/e of its valueat the interface between cladding 2 and coating 4'. The value of d_(p)is given by the relation

    d.sub.p =(λ.sub.e /n.sub.2)/2π[sin.sup.2 θ-(n.sub.p /n.sub.2).sup.2 ]1/2 cm                                   (11)

where θ is the angle of incidence of the light rays on the interfacebetween cladding 2 and layer 4' (this layer can legitimately be regardedas a second cladding). Only a small fraction of the intensity of theincident light will be absorbed per reflection but, by a proper choiceof the concentration of the fluorescent dye, virtually the entireintensity of the light leaving the core at angles greater than θ₆ can beabsorbed over the length L of the resolvable fiber segment. Thegenerated fluorescence can be collected by the optical fiber with highefficiency, because the angular distribution of the fluorescence willfavor the modes within the acceptance angle of the fiber (Lee et. al.,Appl. Opt. 18, 862 (1979))

3.3. A Practical Device for Measuring Distributed Forces

Distributed forces can be measured with the same electro-opticalarrangement as that described with reference to FIG. 4, with the sensingfiber 13' being either one of the fibers illustrated in FIGS. 7 or 8.The light source 10 is driven to produce interrogating light pulses ofsubmicrosecond duration and wavelength λ_(s) within an opticalabsorption band of the fluorescent centers (fluorescent molecules orions) in the applicable cladding. These light pulses are launched intothe core of the sensing fiber, where they propagate along the fiberlength. At any point along the fiber on which a mechanical force isacting, a fraction λ of the intensity of the interrogating light pulsesis deflected out of the core and into cladding 2, and is immediatelyconverted into fluorescence light pulses by the fluorescent centers inthe applicable cladding (cladding 3 if the fiber of FIG. 7 is used, orthe evanescent layer of cladding 4' in the fiber of FIG. 8). Both thefluorescence light pulses and the Rayleigh-backscattered light pulses(from the fiber core) travel back to the electro-optical unit and,through the fiber optic coupler 12 and fiber segments 14 and 15, tophotodetectors 16 and 17, which are made spectrally selective to theforce-dependent fluorescence pulses and the Rayleigh-backscatteredpulses, respectively. The ratio of the photo-electric signals from thetwo photodetectors is an indicator of the value of α and, hence, of themagnitude of the force. The time of arrival of the optical pulses t thephotodetectors, relative to the time of launching of the interrogatinglight pulses, identifies the location of the force. The fluorescencelight pulses are easily separable from the Rayleigh-backscattered pulsesbecause of their different, generally longer, wavelengths.

Suitable Dyes and Cladding Materials for the Practice of This Invention

In principle, any fluorescent dye dissolved in a clear cladding can beused in a force- or temperature-sensing fiber according to the teachingsof this invention. In practice, the choice of dye to be incorporated inthe cladding would be determined by factors including the fiber length,dye properties including fluorescence efficiency, photo-chemical andthermal stability, and solubility in the cladding material. It should beclear to those of ordinary skill that the dye concentration should benot lower than that needed to generate an easily measurable signal overthe length of the spatially resolvable fiber segment, of the order of 1meter for interrogating light pulses with a duration of 10⁻⁸ seconds,and shorter for shorter light pulses. For fibers of length of severalhundred meters or longer it is preferred to use dyes with absorption andfluorescence bands in the orange, red or near infrared regions, to avoidthe high scattering losses which occur at shorter wavelengths. Dyes ofthe violanthrone family have been mentioned above (section 2.1) and havealso been mentioned in the parent application Ser. No. 711,062, filedMar. 12, 1985, now U.S. Pat. No. 5,004,913 as suitable for use inforce-sensing fibers with a fluorescent cladding. Depending, inter alia,on the length of the fiber probe, preferred dye concentrations rangefrom about 10¹⁴ to about 10¹⁸ molecules/cm³.

Regarding cladding materials in which to dissolve the fluorescent dye,any material from which plastic optical fibers are presently made aresuitable. Examples include but are not limited to polyurethanes,polyacrylates (including methacrylates) and styrenic polymers. One canalso dissolve fluorescent dyes in inorganic glasses without having touse temperatures high enough to degrade the dyes, by using thewell-known `sol gel` process, and the resulting dye-doped glasses can beused as fluorescent claddings.

3.4 Force Sensing Fiber Probes and Techniques Not Requiring WavelengthConversion.

One such fiber probe includes a central core A having an index ofrefraction n₁, a first cladding B having an index of refraction ₂ lowerthan n₁ and a light scattering coefficient much higher than that of thecore, and a second cladding C around and in contact with cladding B andhaving an index of refraction n₃ lower than n₂. The thickness ofcladding B is not much greater than about 15 μm, with a cross-sectionalarea comparable to that of the core. In operation, interrogating lightpulses of submicrosecond duration are launched into the core. Under theaction of a force at any point along the fiber, a fraction of theintensity of each interrogating light pulse is deflected into claddingB, wherein it generates pulses of backscattered light. Because claddingB has a much higher light scattering coefficient than the core, forcesacting on the fiber at any point along its length will increase theintensity of the total intensity of the light backscattered from thatpoint. This increase can be measured by ordinary OTDR techniques.Because of the relatively small thickness of cladding B, at least alarge fraction of the intensity of the light deflected into thiscladding will re-enter the core as core-guided modes, especially withinterrogating light wavelengths longer than 1.0 μm.

3.5. The Measurement of Forces with Optical Fiber Probes Having TwoLight-Guiding Regions of Different Effective Optical path Length.

In the embodiments of force-sensing systems discussed above, I havedescribed how light deflected out of the core of an optical fiber underthe action of a force can be processed into an optical signal separablefrom the light transmitted by the fiber core. In the above embodiments,signal separation is effected by converting the deflected light intospectrally separable light of different wavelengths from those of theinterrogating light. Separation can also be effected in the time domain,by processing the pulsed or AC-modulated light deflected out of the coreinto a light having different temporal characteristics from those of theinterrogating light carried by the fiber core, without the need forwavelength conversion. A preferred embodiment of such a technique isdescribed in the following paragraphs, with reference to FIGS. 9 and 9A.

The optical fiber 13A shown schematically in FIG. 9 comprises a singlemode core 1 with an index of refraction n₁, a first cladding 2A aroundthe core having an index of refraction n₂ such that the value of (n₁ ²-n₂ ²)^(1/2) does not exceed 0.15, a light-guiding region 3A aroundcladding 2A, having a graded parabolic or near parabolic index ofrefraction the peak value of which, n₃, is several percent higher thann₁, and an outer cladding 4A with an index of refraction n₄ lower thann₂. The fiber is used as a distributed force-sensing probe with a devicerepresented schematically in FIG. 9A.

Referring to FIG. 9A, light source LS launches into the fiber core atrain of interrogating light pulses with a duration of the order of 10⁻⁹seconds or shorter, depending on the spatial resolution desired. At anypoint where a lateral force is acting on the fiber, a small fraction ofthe intensity of each interrogating light pulse is deflected out of thefiber core into light-guiding region 3A, where it is trapped by thegraded refractive index distribution of this region and by the outercladding 4A. Because n₃ is substantially higher than n₁, the effectiveoptical path length of region 3A, (that is, its actual length multipliedby its index of refraction) is substantially longer than that of thecore 1, so that the light deflected out of the core into region 3Aarrives at the photodetector 16A at the distal fiber end after aresolvable interval t following the arrival of the interrogating lightpulses carried by the fiber core. This interval identifies the locationat which the deflection occurred, according to the relation

    t=(a/c)(n.sub.3 -n.sub.1) seconds                          (12)

where

z is the fiber distance to the photodetector of the point where the theforce was acting; and

c is the velocity of light in a vacuum.

For example, if n₃ is equal to 1.553 and n₁ is equal to 1.46, then twosensing points one meter apart along the fiber will produce signalsarriving at the photodetector about 3.1×10⁻¹⁰ seconds apart, assumingthat the duration of the interrogating light pulse is not longer, andthat region 3A does not introduce a serious pulse broadening. Since thelight-guiding region 3A has a parabolic or near parabolic index profile,the broadening can be shown to be relatively low for some practicalfiber lengths. The pulse broadening Δt is given approximately by therelation

    Δt≈[(n.sub.3 -n.sub.1)/n.sub.3 ].sup.2 [n.sub.3 z/20.c.3.sup.1/2 ]seconds                                 (13)

For a z distance of 100 meters and the above assumed values for n₃ andn₁, Δt is approximately equal to 5.4×10⁻¹¹ seconds. The actualdispersion may be somewhat greater because of some dispersion in thefirst cladding 2A, but spatial resolutions of the order of 1 metershould be obtainable in fiber lengths of about 100 meters. If numerousforces are acting at different locations along the fiber, their signalswill arrive at the photodetector at different resolvable times. Theintensities of the time-resolved signals will be indicators of themagnitude of the forces.

Another embodiment of a fiber with two light-guiding regions ofdifferent optical path length consist of a fiber wherein one of the twolight-guiding regions has an actual path length different, for the sameunit length of the fiber, from that of the other light-guiding region.An example of such embodiment is a fiber as shown schematically in FIG.9B, having a helical core 1A with an index of refraction n₁, a firstcladding 2B around said core, having an index of refraction n₂ lowerthan n₁, and a second cladding 3B around the first cladding, with anindex of refraction n₃ lower than n₂. When light pulses with a durationof 10⁻⁹ seconds or shorter are launched into the core of this fiber, anda lateral force deflects a fraction of the intensity of this light intocladding 2B, the cladding light pulses arrive at the photodetectorbefore the light pulses propagated by the core. The interval t' betweenthe core pulses and the cladding pulses follows the relation

    t'=(a/c)(n.sub.1 C-n.sub.2) seconds                        (14)

where C is the ratio of the actual optical path length of the helicalcore to that of cladding 2B. Another embodiment uses a straight core anda helical cladding around the core.

Optical fibers having a helical light-guiding region tend to lose lightat points other than those at which the forces to be measured areacting, so they are suitable for use in lengths not exceeding about 100meters. For short fiber lengths, fibers with a helical light-guidingregion have the advantage of better spatial resolution than fibers withboth light-guiding regions straight, for the same duration of theinterrogating light pulses.

4.0. The Measurement of Distributed Forces and Temperatures on a SingleUnbroken Optical Fiber.

The fibers illustrated in FIGS. 7 and 8 can be used for measuring bothforces and temperatures independently of each other, not only on thesame fiber, but also on the same location on the fiber. A preferredembodiment of an OTDR device for carrying out such measurements isillustrated in FIG. 10. The sensing fiber 13B is the fiber of FIG. 8,except that cladding 4' has two dyes dissolved in it, A and B, only oneof which, dye A, has a temperature-dependent absorption coefficient wheninterrogated with light of wavelength λ_(v). Dye B is used as areference as taught in section 2.1.1. Temperature distributions areprobed by launching short interrogating light pulses of pre-selectedwavelength λ_(v) from light source 10A, through fiber optic couplers 12Aand 12B, into both the core 1 and the first cladding 2 of the sensingfiber, and the temperature distributions are determined as alreadydescribed (with reference to FIG. 4). Force distributions are probed byinjecting, into the core 1 only of the sensing fiber, similarly shortinterrogating light pulses (with a duration of, for example, 10 to 30nanoseconds, depending on the fiber length resolution desired) ofpre-selected wavelength λ_(s) corresponding to a photon energy not lowerthan the energy E_(s) of the lowest vibrational sublevel of the emissivelevel of dye A, at which light absorption is much stronger than at thewavelength λ_(v) and is essentially independent of temperature over thetemperature range of operation. Any cladding modes of the λ_(s) lightinjected at the launch end of the sensing fiber will be essentially`stripped` by dye A in cladding 4', because of its much higherabsorption coefficient for for light of this wavelength than for λ_(v)light. The intensity of the fluorescence pulses generated at any pointalong the fiber by the force-probing pulses will then indicate themagnitude of the force on the fiber at that point, essentiallyindependent of temperature. Photodetector 16 receives the fluorescencesignals from dye A generated, alternately, by the λ_(s) force-probingpulses and the λ_(v) temperature-probing pulses. Photodetectors 17 and17A receive and measure, respectively, the Rayleigh-backscattered pulsesfor normalizing the force readings, and the dye B fluorescence pulsesfor normalizing the temperature readings.

5.0. The Measurement of Distributed Forces with an Optical Fiber Usingan AC-Modulated C.W. Light Source.

The ability of the optical fibers of this invention to producerelatively large positive signals from forces and/or temperaturesaffords a simple alternative to conventional Optical Time DomainReflectometry (OTDR), which does not require the use of very short lightpulses for measuring the location and magnitude of these parameters. Thealternative method if illustrated by FIG. 11. The light source 10, whichcould be an inexpensive light-emitting diode (LED), is driven by acrystal-controlled oscillator 9 at a frequency preferably not lower than100 kHz, to generate a sinusoidally-modulated interrogating light whichis launched through the fiber coupler 12C into the optical fiber 13'.This fiber can be either one of the optical fibers illustrated in FIGS.7 and 8. A small fraction of the intensity of the interrogating light isdiverted by coupler 12A and optical fiber segment 14A into photodetector16. When the interrogating light beam reaches a point P along the fiberwhere a force is acting, a relatively small fraction α of the intensityof the interrogating light propagating along the fiber core is forcedinto the glass cladding, and is then converted into fluorescence lightby the dye dissolved in the plastic coating. A fraction of the intensityof this fluorescence light travels back along the fiber to the fibersegment 15A and photodetector 17. The fluorescence light arriving atthis photodetector has the same time-domain frequency f as that of theinterrogating light, but has a phase shift relative to it, the magnitudeof which depends on the fiber distance from point P to theelectro-optical unit. Photodetector 17 receives, simultaneously,fluorescence light from other points along the fiber, with a phase shiftdifferent for each location. The phase angle Δθ (relative to theinterrogating light) of the fluorescence light generated at any distanceL' from the electro-optical unit is given approximately by the relation

    Δθ=360(2L'·f·n·c.sup.-1) degrees (15)

where

n is the index of refraction of the glass, and

c is the velocity of light in a vacuum.

Now, one can apply a phase shift by means of phase shifter PS, to thephoto-electric signals produced by the group of fluorescence signalsoriginating throughout the whole fiber length, so that the fluorescenceoriginating at one location, (and only one location) produces acomponent of the photo-electric signal which is exactly in phase withthe signal generated at photodetector 16. Now, if one varies the phaseshift, for instance in the manner of a saw tooth ramp, one iseffectively varying the location along the fiber where the generatedfluorescence produces a photoelectric signal in phase with with thesignal from photodetector 16. This component can be separated from therest of the group of photoelectric signals by means of a lock-inamplifier controlled by the photoelectric signal from photodetector 16.By recording the output of the lock-in amplifier as a function of theapplied phase shift one can, therefore, measure the magnitude of theforces acting on the fiber as a function of the fiber distance from theelectro-optical unit.

Readers familiar with RADAR-ranging techniques may recognize the abovemethod as an adaptation of said techniques to the optical fiber systemsof this invention.

The method is not restricted to fluorescent systems. Any system whichproduces distributed positive signals can be operated according to thisphase shift method, including for example the force-sensing fiber shownschematically in FIG. 9, wherein the force signals carried by the region3A are separated from the core light in the time domain.

6.0. Distributed Sensing Using Stimulated Light Amplification in OpticalFibers.

The distributed sensors described hereinbefore using plastic-solublefluorescent dyes should be adequate for most applications where thesensor is not subjected for extended periods to temperatures in excessof 150 degrees Celsius. If higher thermal stabilities are required, thenan all-inorganic sensing fiber would be desirable. One perceiveddrawback of inorganic glasses is that none are known which combine ahigh luminescence quantum efficiency with the submicrosecondluminescence decay times conventionally required with OTDR systems. Oneobject of this invention is to provide a method and its associateddevices for performing distributed sensing with glass fibers doped withrare earth ions, with fiber length resolutions of a few meters or lessdespite decay times of their spontaneous luminescence longer than 10⁻⁴seconds. The method is illustrated in this section with trivalentneodymium [Nd(III)] as an example, but other glass dopants capable ofstimulated emission should also be useful according to the teachings ofthis section.

Nd(III)-doped glass has a high luminescence quantum efficiency, opticalabsorption bands in the 800-900 nanometer (nm) region (for whichpowerful laser diodes and LEDs are readily available), and luminescencebands from about 900 to 1330 nm. Additionally it has an importantproperty which compensates for its long luminescence decay time, namelythe ability to generate stimulated emission from its excited ⁴ F_(3/2)level. If the optical excitation of Nd(III) is so intense that theoccupancy number of this excited level exceeds the occupancy number ofthe lower level ⁴ I_(11/2) by a number ΔN (a condition known aspopulation inversion), then light propagating through the system with awavelength λ_(f) equal to a laser wavelength λ_(L) of Nd(III) and anintensity I_(o) will be amplified to an intensity (I_(o) +ΔI), themagnitude of (ΔI/I_(o)) being determined at least in part by ΔN. If thepopulation inversion is achieved in a time of 10⁻⁷ seconds or shorter,then ΔI will reach a peak in a comparable period, regardless of thedecay time of the spontaneous luminescence of Nd(III). The shortresponse time permits the use of such a system for distancedetermination with an OTDR. The amplification process is illustrated bythe model of FIG. 12. A `pump` pulse of wavelength λ_(s) within anoptical absorption band of Nd(III) generates the population inversion,which amplifies light of wavelength λ_(L) by stimulated emission. Inthis invention, the absorbed intensity of the pump pulse is determinedby the value of the measurand (force or temperature). The use of fiberswith cores of very small diameter (less than 10 micrometers) permits theproduction of the high excitation densities needed for populationinversion with relatively low power laser sources, even when the laserpower has to be shared by numerous multiplexed sensing points. Examplesfollow.

6.1. A Distributed Temperature Sensor Using a Nd(III)-doped Glass Fiber.

A preferred embodiment is illustrated in FIG. 13. The `pump` lightsource 10A is a pulsed laser emitting a beam of wavelength λ_(v) outsidethe range of wavelengths which can be amplified by an invertedpopulation of Nd(III) ions, and which is selectively absorbed by Nd(III)ions occupying a thermally excited level with an energy K_(v) above theground Nd(III) level. For measuring temperatures above about -100° C.λ_(v) can be selected between about 920 and 980 nm, corresponding toZ_(v) values between about 430 and 1030 cm⁻¹, depending on thecomposition of the base glass in which the Nd(III) ions are dissolved.The long sensing fiber 13C has a core made of Nd(III)-doped glass, witha diameter preferably not greater than 10 micrometers and a dopantconcentration such that the optical density of the fiber at ordinarytemperatures, over its whole length and at the wavelength of theinterrogating (pump) light pulses, is between 0.1 and 0.3 (correspondingto attenuations between 1.0 and 3.0 dB), the fiber length chosenaccording to the particular application. The pump pulses, with aduration preselected according to the spatial resolution desired, and anenergy preselected to generate population inversions along the fiber attemperatures within the range of interest, are launched into the fibercore, wherein a fraction α_(v) of their intensity is selectivelyabsorbed by Nd(III) ions occupying the thermally excited level of energyE_(v). The value of α_(v) is a function of temperature approximatelyaccording to equation (3) in section 2.1 above. The higher the value of60_(v), the greater the population inversion at the Nd(III) level ⁴F_(3/2) relative to level ⁴ I_(11/2), and the greater the amplificationof the intensity of the counterpropagating light of the laser wavelengthλ_(L) (FIG. 12).

For measuring temperature distributions one injects into the same fibercore, but at the distal end, a continuous, moderate power (≦25 mW) laserbeam from a Nd(III)-doped glass fiber laser 25 pumped by a laser diode26. Fiber laser 25 has the same base glass composition as the longsensing fiber (except for a higher dopant concentration), so that itslaser wavelength (near 1.08 micrometers) is essentially the same as thatof the sensing fiber 13C. The latter is `pumped` with a power highenough to produce stimulated emission. Now, as the temperature of anysegment along the sensing fiber increases from an initial value of T₁ toT₂, the number of Nd(III) ions excited to the ⁴ F_(3/2) level increasesfrom N₁ to N₂ approximately according to the relation

    N.sub.2 /N.sub.1 =exp[E.sub.v (T.sub.2 -T.sub.1)/kT.sub.1 T.sub.2)]

when excited with pump pulses from laser 10A. When the number of Nd(III)ions in the excited ⁴ F_(3/2) level exceeds the number in the lowerlevel ⁴ I_(11/2) (FIG. 12) the counterpropagating light from the fiberlaser 25 is amplified. The intensity gained, ΔI, will produce a pulsedsignal arriving at the photodetector 16 in a time, relative to the timeof launching of the excitation (pump) laser pulse, which is a knownfunction of the location along the fiber from which the amplified pulseoriginated, and an intensity which defines the temperature of that fibersegment. In order to correct the signal for any fluctuations of theintensity of the interrogating light pulses, one can monitor theintensity of the Rayleigh-backscattered light from the same fibersegment with photodetector 17. Both photodetectors 16 and 17 are madespectrally selective to the amplified laser light and theRayleigh-backscattered light, respectively, by means of narrow bandpassoptical filters.

At temperatures lower than about -100° C. one has to reduce the value ofE_(v). This is done simply by decreasing the wavelength of theinterrogating (pump) light. It is preferable to use wavelengths whichexcite the ⁴ F_(5/2) level rather than the ⁴ F_(3/2) level, as this willavoid the use of excitation wavelengths which could be amplified in thesame direction as that of the interrogating light pulses (which wouldeffectively compete with the amplification of the counterpropagatinglight of wavelength λ_(L)). Depending on the temperature range beingmeasured, one may select an interrogating light wavelength between about810 and 840 nm. Instead of short excitation pulses, one may useAC-modulated AlGaAs laser light, and measure temperature as a functionof fiber distance to the electro-optical unit by the phase angledivision multiplexing method disclosed in section 5.0, with the addedfeature of the counterpropagating C.W. laser beam from the light source25 in FIG. 13.

An important advantage of the use of light amplification in thesubmicrosecond time domain is that thermal quenching should not operateto a major extent even at temperatures of about 250° C. (and maybe evenhigher), provided that the quenching is due mainly to the shortening ofthe spontaneous luminescence decay time.

6.2 The Measurement of Both Force and Temperature Distributions on aSingle Optical Fiber Probe

An optical fiber having two light-guiding regions of different opticalpath length and a luminescent dopant in one of these regions can be usedas a probe for measuring both distributed forces and distributedtemperatures according to the teachings of this invention. An example ofa preferred embodiment of a device for measuring these distributedparameters is shown schematically in FIG. 14. The fiber 13D differs fromthe fiber shown in FIG. 9A only in that the glass core is doped withNd³⁺ ions in a concentration sufficient to sustain a populationinversion sufficiently high to produce amplification of a probing lightbeam of wavelength near 1.06 micrometers. Temperature measurement withthis fiber is based on the principles set forth in sections 2.1 and 6.1above. The light source 10A launches into one end of fiber 13Dinterrogating light pulses of pre-selected wavelength λ_(v) betweenabout 920 and 1000 nm, depending on the temperature range beingmeasured, and with an energy sufficient to generate atemperature-dependent population inversion of the ⁴ F_(3/2) level ofNd³⁺. At the other fiber end, a CW neodymium glass fiber laser 26launches a CW probing laser beam of wavelength λ_(f) of about 1.06micrometers. As this probing light beam traverses a fiber segmentcontaining a temperature-dependent population inversion, it will bepulse-amplified at that point. The time of arrival of the amplifiedlight pulses at photodetector 16 identifies the location along the fiberwhere the pulse amplification took place, and the magnitude of the gainwill be an indicator of the temperature of the segment.

Force distributions are measured according to the method described insection 3.2 above. The light source 10 launches into the core of opticalfiber 13D at the end opposite to the one on which the interrogatinglight pulses of wavelength λ_(v) were launched, a train of interrogatinglight pulses of a wavelength λ at which the Nd³⁺ ions are transparent.At any point along the fiber where a force is acting, a fraction α ofthe intensity of each pulse is deflected into the fiber cladding, whichhas an optical path length substantially longer than that of the core.The pulses of deflected light arrive at photodetector 16A after aninterval Δt following the arrival of the undeflected interrogating lightpulses, the value of Δt being determined approximately by equation (12)above, which defines the position along the fiber where the force wasacting. The intensity of the deflected pulses is an indicator of themagnitude of the force. Radiometric operation is achieved by dividingthe electrical signals generated at photodetector 16A by those ofphotodetector 17 generated by the Rayleigh-backscattered light from thesame point where the force was acting.

6.3. Other Arrangements for the Sensing of Distributed Forces andTemperatures Using Laser-Active Optical Fibers.

The sensing principles described above can be used for sensing forcesand temperatures distributed over a multiplicity of locations, using asingle laser-active optical fiber probe and one of the arrangementsshown in FIGS. 13 and 14. The following are examples of suitableembodiments of optical fiber probes:

EMBODIMENT 1

FIG. 15 illustrates an embodiment based on a two-core fiber. The fiber13E includes a clear glass core C1, a second core C2 made of glass dopedwith Nd(III), a first cladding 2A common to and around said two cores,and a second cladding 4A around the first cladding. Core C2 haspreferably an index of refraction higher than that of core C1. Cladding2A has an index of refraction slightly lower than that of core C1, andcladding 4A has an index of refraction substantially lower than that of2A. The diameters of the two cores are preferably smaller than 10micrometers. Force distributions are probed by launching interrogating(pump) light pulses having a narrow range of wavelengths λ_(s) into coreC1. A fraction α' of the intensity of this light is ejected from core C1under the action of a force at any point, and is absorbed in core C2,where it generates a population inversion as described hereinbefore, andpulse amplification of counterpropagating light of wavelengths λ_(f) ofabout 1.08 micrometers, the magnitude of which increases with themagnitude of the population inversion and, hence, of the magnitude ofthe force. The time of arrival of the amplified pulses to the measuringphotodetector indicates the location along the fiber where the force wasacting. The wavelengths λ_(s) can be, for example, near 800 nm.Temperature distributions are probed by launching into core C2interrogating light pulses of wavelengths λ_(v), for example centered at946 nm, and proceeding as described above in section 6.1.

Although FIG. 15 shows a circular cross section for the claddingsurrounding the two cores, other cross sections may also be used, forexample an elliptical one. It is not necessary for the two cores to beof similar diameter.

Since core B has an index of refraction higher than that of core A,distributed force sensing can be effected by means other than thegeneration of a population inversion in core B. If the fiber isinterrogated with light pulses injected into core A having wavelengthsoutside the absorption bands of any dopant in core B, a lateral forceacting on the fiber can couple a fraction of the intensity of eachinterrogating light pulse to core B, wherein it will propagate to thefiber distal end with a lower velocity than that of the interrogatinglight pulse within core A. The light thus coupled at any point along thefiber will then arrive at the fiber distal end after a resolvableinterval t following the arrival of the pulse propagating along core A.This interval identifies the location along the fiber under the actionof the force, according to the same relation (12) introduced in section3.2 above. Good spatial resolution can be achieved if both cores A and Bare single mode. From elementary fiber optics theory, core B can besingle mode and still have a substantially higher index of refractionthan core A if its diameter is smaller than that of core A.

EMBODIMENT 2

An alternate arrangement to two cores within a common cladding is a`composite core`, namely a core comprised of two regions of differentchemical composition and index of refraction, but so characterized thata light beam launched into either region at a fiber end propagates atall points along both regions, the optical power distribution betweenthese regions at any point along the fiber being a function of thephysical environment acting on the fiber at that point. An example ofsuch a fiber is illustrated in FIG. 16. The probe is a long opticalfiber 13F having a segmented light guide having a central core A with adiameter of about 5.0 micrometers (μm), an index of refraction n₁, andmade of glass comprised of material laser-active at wavelengths λ_(f1).Around this central region there is a second region B having a diameterof about 8.0 micrometers, an index of refraction n₂ only slightly lowerthan n₁ and no laser-active material, or a material laser-active atwavelengths λ_(f2) different from λ_(f1). Around region B there is athin cladding C with an index of refraction n₃ substantially lower thann₂. Around cladding C there is an outer cladding D with an index ofrefraction n₄ higher than n₃ and only slightly lower than n₂. Regardingits index profile, the fiber is essentially a single mode "W" fiber witha segmented core, where region A is the central part of the core andregion B is the other segment.

If the value of [n₁ -n₂ ] (henceforth referred to as Δn) is lower than0.01, then an appreciable fraction α of the intensity of theinterrogating light launched into core A (or into the light guidecomprising both regions A and B) will propagate along cladding B even inthe absence of any external force on the fiber, due to the penetrationinto cladding B of the evanescent field of the guided light beam. Thecalculated optical power distribution between regions A and B for thelowest order modes of the core A has been determined previously as afunction of Δn, for several values of the radius of the core A [U.S.Pat. No. 4,151,747], and is shown in FIG. 17. These lowest order modesare the only modes in the fiber of the instant example, at sufficientlylow Δn values. It is apparent that small changes of Δn produce arelatively large change in the value of α'. Relatively weak lateralforces produce the same effect, due to the low value of (NA)₁. Since thefiber is essentially a "W" fiber, strains or other forces which maycause a large extent of optical power redistribution between regions Aand B will not cause excessive light losses from the segmented corecomprising these two regions.

If the indices of refraction n₁ and n₂ have different temperaturecoefficients, then the intensity distribution of the interrogating lightbetween regions A and B will be temperature-dependent, and the fiber maybe used as a sensitive distributed temperature probe. From the data ofFIG. 17 it can be verified that a difference in temperature coefficientsof n₁ and n₂ of the order of 10⁻⁵ per kelvin (encountered in practice insome commercial glasses) can lead to temperature coefficients of themeasured signal of the order of one percent per kelvin, adequate formost industrial purposes.

Because of the very low values of (NA)₁ that can be implemented in thefiber without excessive light losses, the optical power distributionbetween regions A and B can be made a very sensitive function of strain,and the fiber may be used as a sensitive distributed force sensor.

The chemical compositions of regions A and B should be chosen accordingto whether the fiber is to be used as a temperature probe or a force(strain) probe. If a force probe, the value of (NA)₁ should preferablybe temperature-independent. This should be easy to ensure, as bothregions A and B could be made of glasses with similar temperaturecoefficients for n₁ and n₂. If a temperature probe is desired, thecompositions of the two glasses are preferably chosen so as to maximizethe change in Δn for a given temperature change.

6.4. The Sensing of Distributed Physical Variables Based on StimulatedLight Amplification in Optical Fibers Having at Least Two Light-GuidingRegions, Using Backward-Stimulated Raman Scattering (BSRS) Processes.

Raman scattering is just one form of light scattering, and it differsfrom other forms of light scattering in that, when generated bymonochromatic light or light of narrow spectral bandwidth of wavelengthsλ_(s), Raman-scattered light has wavelengths λ_(f) substantiallydifferent from λ_(s), the difference in photon energy correspondingusually to the energy of a vibrational quantum. Brillouin-scatteredlight also involves a spectral shift, but a much smaller one.

The above-captioned technology was disclosed in co-pending applicationSer. No. 102,835, where it was briefly discussed (page 30, paragraphbeginning on line 4) as an alternative to the use of a luminescent glassas amplifying medium. It was further discussed in preceding paragraphsof this application, and is discussed in more detail in this and thefollowing paragraphs. A preferred embodiment of a distributed fiberprobe based on BSRS is illustrated in FIG. 16, as described above, whereone of the two light-guiding regions A or B is comprised of materialcapable of stimulated Raman scattering.

The fiber probe can be used in essentially the same device configurationas shown in FIG. 13, described with reference to a rare earth-dopedlaser-active fiber. The main difference is that stimulated lightamplification in the BSRS probe occurs by stimulated Raman scatteringinstead of stimulated luminescence as in the rare earth-doped fiberprobe. In the BSRS probe the wavelength λ_(f) of the counterpropagatinglight beam generated by the light source 26, instead of beingindependent of the pump wavelengths, should follow approximately therelation

    λ.sub.f.sup.-1 =λ.sub.s.sup.-1 -Δν

where λ_(s) is the wavelength of the interrogating light pulses and Δνis the quantum energy of the Raman Stokes shift. The word "Backward"before "Stimulated Raman Scattering" indicates that the stimulated Ramansignals received by the photodetection station originate from the lightinjected into the fiber at the end opposite to that where theinterrogating `pump` pulses are injected. In one example of a BSRS fiberprobe having a cross-sectional structure as shown in FIG. 16, the core Ais comprises of glass with a high concentration of P₂ O₅, and issurrounded by a region C which does not contain a significantconcentration of P₂ O₅. The magnitude of the Stokes shift is about 1,200cm⁻¹. In a preferred embodiment, the pump laser 10A launches into thelight guide comprising regions A and B interrogating light pulses of anintensity sufficient to generate stimulated Raman scattering at allpoints along the fiber, and a duration τ of approximately [(n₁ +n₂)L/c]seconds, where L is the spatial resolution desired (also the interactionlength of the pump pulses with the counterpropagating light ofwavelength λ_(f)) and c is the velocity of light in free space. As theprobe wave traverses the interaction length L of the fiber, itsintensity I_(a) within region A is pulse-amplified to the intensityI_(s) according to the relation

    (I.sub.s /I.sub.a)=exp[gP.sub.o (1-α')L]             (16)

where

g is the stimulated Raman gain coefficient of the doped glass; and

P_(o) is the power density of the pump pulse over the interaction lengthL.

If the value of the exponent in equation (16) is not much greater thanabout 0.10, then ΔI, the increase in the magnitude of I_(a), is givenapproximately by

    ΔI/I.sub.o ≈g·P.sub.o (1-α').sup.2 L (17)

where I_(o) is the intensity of the probe wave propagating along bothregions of the segmented core over the interaction length L. Note thatthe fractional quantity (1-α') is squared. This arises from the factthat a force which redistributes the pump power between regions A and Balso redistributes the power of the counterpropagating probe wave inapproximately the same ratio.

To give an idea of the magnitude of ΔI one can obtain from thistechnique, let us assume some typical values for the parameters ofequation (17), as follows:

I_(o) =10 milliwatts (mW)

P_(o) =5×10⁷ W/cm² (about 10 W per cross-section of region A)

g=1×10⁻¹⁰ cm/W (about the value of g for GeO₂ at 1.06 μm)

L=100 cm.

α'=0.5

Diameter of region A=5 μm

Then the magnitude of ΔI is about 1.25 mW, more than adequate foraccurate measurements. By increasing the pump pulse intensity anddecreasing its duration, one could obtain spatial resolutions of theorder of centimeters.

While the fiber in the above example uses P₂ O₅ as the Raman-scatteringmaterial, other materials, including many known to have higher Ramanscattering coefficients, are also suitable for the purposes of thisinvention. Especially useful are glasses containing heavy metal oxides(see, for example, the publication by Miller et. al., Journal ofNon-Crystalline Solids 99, 289-307 [1988]).

7.0 Miscellaneous Applications of the Invention

7.1 Optical Tactile Sensors for Robots.

The teachings of this invention lend themselves to the construction ofnew useful devices. The same principles described above for themeasurement of both forces and temperatures with a single sensor can beused to construct a two-dimensional tactile sensor for robots, alsosensitive to both forces and temperature. The sensor can be understoodwith reference to FIG. 18. It consists of a clear elastomeric "sandwich"pad comprising three layers 31, 32, and 33. The inner layer 31 has anindex of refraction n₁, and is bounded by the upper layer 32 and thelower layer 33, having indices of refraction n₂ and n₃, respectively,such that n₁ >n₂ >n₃. In one embodiment layer 31 has a thickness ofabout 200 micrometers, while each of the layers 32 and 33 have athickness of about 100 micrometers. Layer 32 has dissolved therein afluorescent dye characterized by high fluorescence quantum efficiencyand an optical absorption coefficient which is a sensitive function oftemperature when illuminated with light of wavelength λ_(v). Whenilluminated with light of a wavelength λ _(s) shorter than λ.sub., theabsorption coefficient of the dye is independent of temperature withinthe temperature range of operation of the device.

The fluorescent layer has an electrically conductive film 34 applied toit, having a resistivity such that a relatively low current passedthrough it will heat layer 32 to a a temperature of about 40° C., or toany other chosen temperature.

The outer faces of layers 32 and 33 are coated with strips 35 of blackpaint as shown in the top view of FIG. 13, with a width at least tentimes greater than the thickness of layer 31.

The whole pad is in contact, through layer 33, with a two-dimensionalsilicon imaging array 36, having a light-sensitive surface covered witha dielectric optical filter 37, which is selectively transparent to thefluorescence wavelengths of the dissolved dye, and blocks the wavelengthλ_(s) of the illuminating light source.

The sensor pad works as follows:

Light of wavelength λ_(s) is injected into layer 32, uniformly from oneor, preferably, two or more square sides. The injected light has anangular distribution such as to overfill the numerical aperture (NA) oflayer 31 defined by the relation

    (NA)=(n.sub.1.sup.2 -n&hd 2.sup.2).sup.1/2

The light rays having angles smaller than the critical angle θ_(c) fortotal internal reflection enter layer 32 and are `stripped` by the blackpain strips. The light reaching the clear inner square of the pad ispropagated inside layer 31 by total internal reflection at theboundaries between layers 31 and layers 32 and 33. Any pressure at anylocalized point 38 on the pad causes a deformation which forces afraction of the intensity of the light propagating thorough layer 31into layer 32, thus causing a fluorescence emission from the dissolveddye, which is detected by the detector(s) in the imaging array locatedjust below the point 38. The intensity of the generated fluorescence isdetermined by the magnitude of the deformation and, hence, by themagnitude of the pressure at that point. If pressure is applied at aplurality of points within the clear area of the pad (that is, withinthe area bordered by the black strips) the silicon sensor array willgenerate a two-dimensional map of the forces acting on the pad.

After a video frame is obtained, the process is repeated, except thatthe light injected into layer 31 now has the wavelength λ_(v). Thiswavelength will produce a fluorescence intensity from the dye which isdetermined by the temperature of layer 32 as well as by the pressureapplied at point 38. The ratio of the fluorescence intensities producedby the two illuminating wavelengths defines the temperature at point 38,while the fluorescence intensity produced by the illuminating wavelengthλ_(s) defines the magnitude of the pressure or force.

7.2 Fiberoptic Keyboards

The British journal Sensor Review (April 1984, pp 70-72) discusses theneed for electrically passive fiberoptic keyboards in some environmentswhere electronic keyboards may be unsuitable, for example in underwaterapplications, in flammable atmospheres, or in electrically noisyenvironments, and describes a complicated experimental fiberoptickeyboard. The teachings of this invention for the trapping andmodification of light deflected out of the core of an optical fiberunder an acting force can be used to construct improved, simplerkeyboards. In contrast to the device described in said issue of SensorReview, which require 2·N^(1/2) sequentially pulsed light sources for anumber N of keys, the fiberoptic keyboards of this invention require notmore than one light source. Furthermore, this single light source can beshared by numerous fiberoptic keyboards operated simultaneously.

An example of a fiberoptic keyboard using the principles of thisinvention uses two identical optical fibers, each comprising, a) a clearcore having an index of refraction n₁ ; b) a clear cladding around saidcore, having an index of refraction n₂ lower than n₁ and a thickness ofa few micrometers; c) a second cladding having an index of refraction n₃not lower than n₂ and a diameter preferably greater than twice the corediameter and having also a wavelength-selective light absorber dissolvedtherein; and d) an outer cladding with an index of refraction n₄ lowerthan n₂.

A general representation of the optical transmission spectrum of asuitable light absorber is shown in FIG. 19, which also includes atypical output spectrum of a C.W. light-emitting diode (LED) used foroperating the keyboard, including at least two easily separablewavelengths λ₁ and λ₂. In the keyboard system, a length L of each fiberis used such that the total optical density to light of wavelength λ₁propagating along the light-guiding region including the light-absorbingsecond cladding is about 1.3, corresponding to a transmitted lightintensity P of about 5 percent of the incident light intensity P_(o),according to the relation

    log(P/P.sub.o)=-αL

where α is the absorption coefficient of the light-absorbing secondcladding per unit length, the product αL being the full-length opticaldensity of the fiber.

FIG. 19A is a schematic representation of the operating keyboard. Twofibers, F1 and F2, are laid out as shown under an 8×8 matrix of keys, F1passing under all rows of keys and F2 passing under all columns of keys,the two fibers intersecting under each key as shown in the area ofdetail under the matrix. Between each contiguous rows and columns ofkeys there is a fiber segment S of length z, at least 10 times longerthan the fiber segment under each row or column of keys. When a key isdepressed, at least an appreciable fraction of the intensity of theinterrogating light launched into the core of each fiber by the LED isdeflected into the light-absorbing second cladding. The deflected lightpropagates within the light-guiding region including said secondcladding along the fiber length to the photodetection station PS, andthe spectral component of wavelength λ₁ is partially absorbed. Thetransmitted light intensity P_(i) of wavelength λ₁ is given by therelation

    log(P.sub.i /P.sub.o)=αL(1-N'z'-x)

where

z' is equal to z plus the length of fiber under a row or column of keys,

N' is the number of rows or columns (depending on whether the fiber isF1 or F2, respectively) preceding the row or column of the depressedkey; and

x is the length of fiber under the preceding keys of the same row orcolumn of the depressed key.

Since z' is more than 10 times greater than x, the value of x,determined by the number of preceding keys in the same row or column,won't affect appreciably the measured value of P_(o). Additionally,light of wavelength λ₂ is not absorbed appreciably by thelight-absorbing material. Therefore, the ratio of the light intensitiesof wavelengths λ₁ and λ₂ transmitted by each fiber will be a uniquefunction of the location of the depressed key. The light outputs fromeach fiber are fed to two photodetector pairs, PD1-PD2 and PD1'-PD2',the digits 1 and 2 representing that the photodetectors are selectivelysensitive to wavelengths λ₁ and λ₂, respectively.

For a full-length optical density of 1.30, the light intensities ofwavelength λ₁ transmitted by each fiber for keys depressed at twocontiguous rows or columns varies by about 30 percent for an 8×8 matrixof keys from one row or column to the next, and the percentage ofvariation increases as the full-length optical density increases.Therefore, the set of readings from the two sets of photodetectors, foreach depressed key, will unambiguously identify the key.

The reading can be made independent of any variation of the fraction ofthe interrogating light intensity deflected into the absorbing cladding,by directing only the deflected light to the photodetectors. This can beachieved simply by covering the output tip of the fiber core by a blackabsorber, or by other means which would be apparent to workers with atleast average competence in the art to which this invention pertains.

In a variant of the above method, the light-absorbing material in thesecond cladding is luminescent, and light of the shorter wavelengthswithin the luminescence spectral output is partially reabsorbed, theextent of the reabsorption varying as a function of the distance fromthe point under the depressed key to the distal end of the fiber, theluminescence emitted at longer wavelengths being only minimallyabsorbed. In this case one identifies the depressed key by the ratio ofthe intensities of the lights emitted at the two wavelength regions. Onespecific example of such a luminescent system is Nd(III)-doped glass,the luminescence spectrum of which comprises a band at about 890 nm, andanother strong band at about 1060 nm (in addition to at least anotherband at longer wavelengths). The luminescence light within the 890 nmband is partially reabsorbed, while the band at about 1060 nm is onlyminimally attenuated, and can be used as a reference.

Another kind of keyboard, suitable for multiplexing, is illustrated inFIG. 19B. A light source 10, preferably an LED or a laser diode, isdriven from power supply 9 to produce a recurrent train of light pulseswith a duration preferably not greater than about 20 nanoseconds, and arepetition rate of 10⁴ pulses per second. At the keyboard the opticalfiber 138 is laid out as shown, with intersecting horizontal andvertical segments, and with a depressable key at each intersection.Between each contiguous rows or columns of keys traversed by fiber 138there is a fiber segment of length L of about 2.0 meters, designed todelay the time of arrival of the interrogating light pulses at any rowor column by about 10 nanoseconds after the preceding row or column. Atany key position, the set of fiber distances from the point ofintersection to the electrooptical unit ECU is unique for that key. Asthe key is depressed, a small (but easily measurable) fraction of theintensity of the interrogating light propagating through the fiber coreis forced into the fiber cladding and the interface between thiscladding and the fluorescent coating 4'. The fluorescence light pulsesgenerated at the evanescent wave layer of the coating arrive at thephotodetector 23 at two specific times (relative to the time ofinjection into the fiber of the interrogating light pulses) whichuniquely identify the key. The power supply 9 provides the referencetiming pulse which triggers the time sweep of the timer TR. The timeinterval between the fluorescence pulses from contiguous rows or columnsof keys is approximately 20 nanoseconds. In the illustrated example, thetotal length of optical fiber needed for 49 keys is 28 meters. Ingeneral, the total fiber length L needed for any number N of keys isgiven by

    L=(tc/n)·N.sup.1/2

where

c is the velocity of light in a vacuum;

t is the needed time resolution; and

n is the index of refraction of the glass in the fiber. (Although theindices of refraction of the core glass and the cladding glass areslightly different, they are sufficiently close to 1.50 to use thisvalue in most design calculations).

The electro-optical unit described above can be used, without majormodifications, to interrogate and read out a multiplicity of keyboardsin a series array on a single fiber, or in parallel on a plurality offibers.

The technique described above for the for the time-division-multiplexedkeyboard can be used with any fiber in which light deflected from thecore into another light-guiding region can be separated in the timedomain from the interrogating light. The fiber 13A subject of FIG. 9 isan example of an optical fiber also suitable for use in fiberoptickeyboards. In this case the light deflected under the action of adepressed key is separated in the transmission mode as discussed insection 3.2

7.3. The Multiplexing of Sensors on a Single Unbroken Optical Fiber.

Distributed force sensors can be used for multiplexing sensors for anyphysical variable which can be converted into a mechanical force. Thedistributed fiber sensors of this invention can receive inputs from anysuch sensor at any desired location, and transform those inputs intoluminescence signals when the optical fiber is interrogated by asuitable light source in a device like an optical time domainreflectometer (OTDR). As explain hereinbefore, the intensities and thetime characteristics of these luminescence signals define both thelocation along the fiber where the sensor is located and the value ofthe measurand. A preferred type of sensor to be coupled to thedistributed sensing fibers of this invention is the well-known microbendsensor. Examples of microbend sensors which can be multiplexed on asensing fiber of this invention are:

(a) microbenders mechanically coupled to a displacement-type sensor, forinstance the diaphragm of a pressure-sensing transducer. Thus the sensorcan be entirely electrically-passive; and

(b) electrically-driven microbenders, in which case only the signaltransmission through the fiber will be electrically passive.

A preferred way of coupling sensor information into an optical fiberaccording to this invention is to convert the sensor output into anoscillatory force applied to the fiber, the oscillatory frequency beinga known function of the value of the parameter being measured. Afrequency signal is transmitted through an optical fiber with lessdegradation than an intensity signal.

7.4. A Fiber Optic Cooler.

When one excites the luminescence of a highly efficient luminescentmaterial with light of wavelengths longer than the median wavelength ofthe luminescence light emitted by the material, as discussed above, thematerial can be cooled, and this can be used to construct a purelyoptical cooler. The concept of cooling by light can be understood withthe help of FIG. 1. Consider a solid photoluminescent material having aluminescence quantum efficiency near unity (or at least greater than0.9), and a molecular electronic energy level diagram representedschematically by FIG. 1. The relative molecular populations of levels40, 41, 42 and 43 follow the Bose-Einstein population factor [exp(E_(v)kT)-1]⁻¹, where E_(v) is the level energy relative to the ground level40. At ordinary temperatures there is a small but operationallysignificant number of molecules of the solid occupying the thermallyexcited level 42 with an energy E_(v) of about 3000 cm⁻¹ above theground level. Now, suppose the photoluminescent material has anelectronically excited emissive level with an energy E_(s) of 15,500cm⁻¹ , and that the mean photon energy of its luminescence light is14,500 cm⁻¹. The wavelength of the excitation light source is 800nanometers (nm), available from the very efficient AlGaAs diode lasers.The emitted photons have an energy 16% higher than the absorbed photons.This means that is the luminescence quantum efficiency is appreciablyhigher than 0.86, the solid body will be cooled.

Practical Considerations.

For an E_(v) value of 3000 cm⁻¹, the optical absorption coefficient ofthe luminescent material will be 6 to 7 orders of magnitude smaller thanits peak absorption coefficient. This means that, for efficientoperation,

(a) The peak absorption coefficient must be high, of the order of 10⁻¹⁷to 10⁻¹⁶ cm² per molecule, and

(b) The optical path along the luminescent material must be long,preferably of the order of several meters or longer.

The above requirements can be met by using efficient fluorescent dyes asdopants in long optical fibers. Dyes with luminescence quantumefficiencies greater than 0.90 are already known.

For practical applications, one could attach the device to e cooled to afiber optic coil containing the fluorescent dopant.

8.0. Non-Imaging Optical Concentration Techniques Relating to thisInvention.

Unlike the embodiments of this invention based on the use of fluorescentdyes, the excitation densities in order to achieve stimulated emissionand probe light amplification in distributed sensing systems. The way toachieve these high excitation densities with excitation sources of lowor moderate power (not more than a few hundred milliwatts) is to confinethe excitation energy into an active wave-guiding region of the sensingfiber having a very small cross section. The emitting areas of manyotherwise convenient light sources are often much larger than suchneeded small cross-sections. Commercial laser diode arrays have outputpowers from 790 to 870 nm of the order of 1 Watt C.W., originating froma rectangular source about 200 micrometers wide and about 1 micrometerhigh. One could clearly not image more than a few percent of this powerinto a fiber core with a diameter of a few micrometers. If, however, thecore contains a luminescent material which efficiently absorbs theoutput of said diode laser array, one could concentrate at least a largefraction of the energy emitted by the array into said core, withoutviolating the second law of thermodynamics, by means other than theimaging of the light source into the core launch end. One way is toenclose the luminescent core within a clear first cladding ofconventional circular cross-section, into which the light source isimaged. A far more efficient arrangement is disclosed herein, using asan example a temperature-sensing optical fiber based on Nd(III)-dopedglass. The principle is shown in the preferred embodiment illustrated inFIG. 20. It uses a fiber 13G with a light-guiding region A, having anindex of refraction n_(a) and a near rectangular cross-section, with itsheight about equal to or not much higher than the diameter of thecylindrical Nd(III)-doped core B, of not more than about 10 micrometers(μm), and a width not much greater than, and preferably equal to, theactive width of the diode laser array (˜200 682 m). The index ofrefraction n_(b) of the doped core is higher than n_(a). Around region Athere is a cladding C with an index of refraction n_(c) lower thann_(a). Light from the laser diode array is launched into region Awherein it propagates and, at each pass along the width of the nearrectangular cross-section, it passes through the doped core, a smallfraction of its intensity is absorbed by the Nd(III) in the core, untilessentially its whole intensity is absorbed in the core. The opticalenergy that can thus be coupled into the core can be many times greaterthan that which can be launched into the core end. Furthermore, the nearrectangular cladding A of FIG. 20 affords more efficient pumping perunit fiber length than an arrangement using a conventional cylindricalcladding of diameter equal to the active width of the diode laser array.In the latter case, if the doped core is at the center, then theso-called `skew` rays, which would outnumber the so-called `meridional`rays, would not enter the doped core and would not contribute to thepumping. And if the doped core is placed near the edge of thecylindrical cladding in order to capture the skew rays, then theexcitation yield per unit fiber length would be many times lower than inthe case of the near rectangular cladding A, as the cross-sectional arearelative to the cross-sectional area of the doped core is much larger.

A distributed force-sensing fiber using the same optical concentrationprinciples must be adapted to meet the requirement of a minimalluminescence background per resolvable fiber length in the absence of anexternal force (or internal strain). Such a fiber, 13H, is illustratedin FIG. 21. The waveguiding region A' into which the interrogating lightis launched has a cross section about an order of magnitude greater thanthat of the Nd-doped core B. Between core B and region A' there is acladding C having an index of refraction lower than those of core B andregion A'. Core B has an index of refraction not lower than that ofregion A'. Around region A' there is a second cladding D having an indexof refraction lower than that of the first cladding C. At any pointalong the fiber where a force is acting, a fraction α_(s) of theintensity of the interrogating light launched into the light-guidingregion A' is ejected into cladding C and core B, where it produces apopulation inversion of the Nd(III) ions. The Nd(III) concentration incore B can be pre-selected so as to cause total absorption of theforce-coupled interrogating light over the resolvable fiber length (forinstance 1 meter). The wavelength λ_(s) of the interrogating light canalso be pre-selected for optimum absorption. The strongest absorptionband of Nd(III) in the near infrared region peaks at or near 805 nm,generated by efficient, commercially available diode lasers, includingdiode laser arrays. One could also use interrogating light pulses withpowers of the order of hundreds of watts or more, and durations of theorder of 10⁻⁷ seconds or less, without generating stimulated-Brillouinscattering or stimulated raman scattering that would occur if the samepowers were launched directly into the fiber core.

Distributed force sensing with the above fiber can be carried outaccording to the phase angle division multiplexing technique discussedin section 5.0 above, using a counterpropagating moderate power (≦25 mW)continuous laser beam to probe the population inversion in the fiberproduced by the action of the forces to be measured. In order for themethod to be applicable, the amplification of the probingcounterpropagating light must occur at the same time domain frequency asthat of the light modulation at the source. This requires that thesmallest forces to be measured cause a population inversion in each ofthe sensing points along the fiber. If there are, for instance, 50 ormore sensing points, it may be necessary to couple into core B anoptical power of the order of 100 milliwatts or more of the AC-modulatedlight from the `pump` laser (or array). The concentration techniquedisclosed herein facilitates this task.

This concentration technique is also valuable for coupling into anoptical fiber an optical power high enough to cause undesirablenon-linear effects or damage if focused directly into the launch end ofthe fiber core.

The optical concentration method of FIG. 20 can also be used for pumpinga fiber laser or amplifier with a light source of larger emissive areathan that of the fiber laser or amplifier.

8.1 An Application; the Pumping of Erbium-Doped Fiber Amplifiers.

The pumping system illustrated in FIG. 20 can be used for constructingpractical pump sources for optical fiber amplifiers useful for longdistance telecommunications. It is known that state-of-the-art silicafibers have their lowest light attenuation for light wavelengths ofabout 1.54 micrometers (um). This also happens to be the spectral regioncomprising the main laser band of Er-doped glass. Thus, Er-doped fiberamplifiers are presently the subject of considerable development by aplurality of laboratories.

It is generally agreed that a (980±10) nm pump source is desirable, butno practical such source is presently available. Considerable effort isunder way to develop laser diodes operating at about 980 nm, but theyare presently very costly and are likely to remain so in the nearfuture. The teachings of this invention allow the use of inexpensiveAlGaAs lasers or laser arrays operating at about 800 to 820 nm forgenerating the pump radiation at about 980 nm inexpensively. A systemfor generating said pump radiation is shown schematically in FIG. 22.The laser diode array LD pumps a fiber laser system FL, which emitslaser radiation of wavelengths near 980 nm. There are at least threedifferent embodiments of the fiber laser system FL, as follows:

EMBODIMENT 1

In this embodiment the fiber laser system FL includes an optical fiberF₂ whose cross-sectional structure perpendicular to the fiber length isillustrated in FIG. 23. The fiber core A, having a radius not greaterthan a few micrometers and an index of refraction n₂, is comprised ofNd³⁺ -doped glass and is surrounded by a thin first cladding B made ofglass doped with Sm³⁺. Cladding B has a diameter of a few micrometersand an index of refraction n₂ lower than n₁. A second cladding Csurrounds cladding B. Cladding C has a near rectangular cross sectionthe larger dimension of which is much greater than diameter of the core,and is made of a glass transmissive to the pump radiation, and has anindex of refraction n₃ lower than n₁. Around cladding C there is anouter cladding D having an index of refraction n₄ lower than n₃. Thefiber F₁ is connected in series to a fiber F₂, whose core is comprisedof Yb³⁺ -doped glass. Pump radiation from the laser diode array havingan intensity high enough to generate a population inversion of the Nd³⁺ions in core A is launched (injected) into cladding C at the proximalend of fiber F₁. After numerous passes through the Nd³⁺ -doped core,most of the intensity of the pump radiation is absorbed by the Nd³⁺ions, generating a population inversion therein, and laser action atwavelengths near 940 nm. Laser action at the main Nd³⁺ laser wavelengthsnear 1060 nm or at the wavelengths near 1400 nm is prevented by the Sm³⁺ions in core B, which selectively absorb these wavelengths and thereforeprevent their amplification, while being essentially transparent tolight of wavelengths near 940 nm and to the 810 nm pump wavelengths. Thelaser radiation from Nd³⁺ pumps the Yb³⁺ ions in the core of fiber F₂.At sufficiently high pump powers from the Nd³⁺ -doped core of fiber F₁,most of the Yb³⁺ ions in the Yb³⁺ -doped core of fiber F₂ will be in the² F_(5/2) excited level, and laser or superluminescence emission atabout 975 nm will occur. This laser or superluminescence emission can beused for efficiently pumping an Er³⁺ -doped fiber amplifier.

The use of wavelength-selective absorbing claddings can be used as ageneral method for suppressing laser action in a fiber laser within anotherwise strong laser transition, thus facilitating laser action in aless strong laser transition of the same laser material.

EMBODIMENT NO. 2

In this embodiment the fiber laser system FL comprises a fiber similarto that shown in FIG. 20. The fiber core is comprised of a glassco-doped with Nd³⁺ and Yb³⁺, both at concentrations of about 1% orhigher. This core, with an index of refraction n₁, is disposed within aclear first cladding of near rectangular cross-section, with its largerdimension similar to that of region A of FIG. 20 and an index ofrefraction lower than that of the core. This cladding is surrounded byan outer cladding like cladding D of FIG. 20, and having an index ofrefraction lower than that of the first cladding. The only significantdifference with the fiber of FIG. 20 is that the core is co-doped withNd³⁺ and Yb³⁺. The radiant output from the laser diode array of FIG. 22is launched into said clear first cladding and, as it propagates alongthe fiber, it is absorbed by the Nd³⁺ ions in the core. The absorbedenergy is transferred to the ² F_(5/2) excited energy level of the Yb³⁺ions, generating a population inversion therein. When the number of Yb³⁺ions occupying this excited level exceeds the number occupying the grouplevel ² F_(7/2), laser light is emitted at wavelengths near 975 nm,which can be used without further transformation to pump an Er³⁺ -dopedfiber amplifier.

EMBODIMENT NO. 3

This embodiment of the fiber laser system FL uses the same fiber F₁ asembodiment No. 1, to generate the same laser radiation at wavelengthsnear 940 nm. The difference is that the fiber is connected in series tofiber F₃, whose core is comprised of material capable of stimulatedRaman-scattering and having a Raman shift Δν of about (450±20) cm⁻¹.Under strong pump radiation as in Embodiment 1, the fiber F₃ will emitstrong radiation at wavelengths near 980 nm, which can be used forpumping an Er³⁺ -doped fiber amplifier.

9.0. Applications of this Invention to Fiber Optic Communications.

The provision of a luminescent waveguiding region to an optical fiber,in addition to the standard glass core and glass cladding ofcommunication fibers, and without affecting the light propagationproperties of these standard waveguiding regions, allows a plurality ofnew communications-related uses of the fiber, besides its use as adistributed sensing probe. Two of such uses are outlined below:

Improved Diagnostics of Fiber Optic Networks.

In its simplest embodiment as a distributed sensing probe, theforce-sensing fiber of this invention is essentially a standardcommunications fiber with a standard core, a standard cladding, and aplastic coating around the cladding which, except for a few parts permillion of a fluorescent dye dissolved in it, would be essentially thesame as some of low index polymers presently used for the protectivecoating of optical fibers. Yet it is that minute concentration of thedye which enables the fiber to produce diagnostic signals from glossypoints along the fiber which are orders of magnitude stronger than thosefrom the decrease of the intensity of the Rayleigh-backscattered signalsconventionally measured with OTDR diagnostic instruments, but withoutaffecting the communication signals propagating along the fiber core.The force-sensing fiber of this invention could, therefore, be used as acommunications fiber in local area networks, and its force-sensingcharacteristics would e a built-in diagnostic feature which wouldgreatly extend the capability of optical time domain reflectometry. Thiscommunications fiber could be used, additionally, for distributedsensing, and the sensor information would be transmitted through thesame fiber without interfering at all with its usual communicationfunctions.

The Non-Invasive Coupling of Information to the Fiber from the Side atAny Point.

The force-sensing fibers of this invention allow the non-invasivecoupling of information into the fiber from the side at any point. Theinformation can be coupled in digital form by any sequence and/or timingof `force bits` applied by means of, for example, a piezoelectrictransducer, when a train of short interrogating light pulses arepropagating along the fiber core. The coupled information, convertedeither to fluorescence light pulses if the fiber has a fluorescentcladding, or to time-resolved light pulses if the fiber is like the onedescribed in section 3.2 supra and FIG. 9 will then propagate to atleast one fiber end time-and/or wavelength-separated from theinterrogating light. Information can also be coupled by direct opticalexcitation of the fluorescent cladding by an external light source.

10.0 A Medical Application: Laser Surgical and Photo-Irradiation TipsWhich Are Also Temperature-Sensing Probes.

In recent years there has been a large increase in the use of high powerlasers in medicine, for applications like surgery, hyperthermia,photodynamic therapy, and other laser irradiation techniques. In onetype of application, laser energy conducted by an optical fiber or fiberbundle to a surgical tip heats up the tip to permit operations requiringcontrolled heating of the biological tissue being worked on.Unfortunately, there is no reliable means for measuring the temperatureof the heated tip. Thermal measurements are usually conducted by placinga temperature sensor at some distance from the tip, but this is not anadequate substitute for measuring the temperature at the tip itself.

The teachings of this invention permit one to make surgical tips whichare also their own temperature probes and which, in addition, canfunction as light diffusers for medical photoirradiation purposes.

An example of a surgical tip/light diffuser according to this inventionis illustrated in FIG. 24. It consists of a length of clad rod 80 ofsimilar diameter to that of the optical fiber to which it is attached,and having dissolved therein a luminescent material with a luminescencespectrum suitable for the intended treatment, if used forphoto-irradiation, at a concentration pre-determined to convert thedesired fraction of the excitation light into luminescence light.

Used as a surgical tip, the rod, which may be terminated in a point asshown, is heated by absorption of laser radiation from sources like anexcimer laser (ultraviolet), an AlGaAs laser array or a carbon dioxidelaser. The laser radiation is delivered through an optical fiber 81, ata power and energy needed for heating the rod to the desired temperaturerange. Suitable based materials for the rod are sapphire and yttriumaluminium garnet, suitably doped with a luminescent material. A suitabledopant for yttrium aluminium garnet (YAG) is Nd³⁺, the doped materialbeing designated herein as Nd:YAG.

The probe can be used as a temperature sensor (usually to measure thetemperature at which it has been laser-heated) by the method disclosedin section 2.1, above. Nd:YAG, for example, can be interrogated withlight of wavelength of 946 nm. Under this illumination, the probeabsorbs a temperature-dependent fraction α_(v) of the interrogatinglight determined by equation (3), with E_(v) being approximately equalto 857 cm⁻¹. The luminescence intensity of the probe will followapproximately equations (4) and (5).

The same probe can also be used as a light diffuser. The Nd:YAG probecan be excited through the fiber 81 by a high power AlGaAs laser diodearray emitting at about 805 nm, within a strong absorption band ofNd:YAG. The absorption energy is converted into luminescence light whichis emitted nearly isotropically from the probe, thus affordinghomogeneous illumination of the biological tissue surrounding the probe.Other luminescent dopants like Cr³⁺, Er³⁺ or He³⁺ can be used forproviding illumination at different wavelengths, depending on thedopant.

11. A Fiber Optic Temperature Control System for `Hot Tip` LaserAngioplasty.

The destruction of tumors and the removal of arterial plaque are growingapplication of lasers. There are two main laser techniques for plaque ortumor removal. One involves direct laser irradiation of the plaque ortumor. A potential danger is the accidental laser irradiation of thearterial wall, which could have serious consequences. The other lasermethod consists of laser-heating a solid tip, and localized thermaldestruction of the tumor or plaque by the tip. This would be thepreferred technique if one could easily measure and control the tiptemperature. Prior art methods are unsuitable because they require atemperature probe external to the heated tip. This may occupy preciousspace in an already tight environment. According to the teachings ofthis invention one can make the heated tip its own temperature probe.This can be achieved by incorporating within the heated tip atemperature-sensing material chosen from the group of luminescentmaterials having a temperature-dependent light absorption coefficient,discussed in section 2.1 above.

A preferred embodiment of a device for controlling temperature in laserangioplasty is illustrated in FIG. 24A. A Nd:YAG laser 10E injects aC.W. laser beam of 1.06 μm wavelength and an optical power P_(o) of theorder of 10 watts or greater, through the input fiber 11 and coupler 12,into the optical fiber 13K. The distal end of the fiber is terminated ina Nd:YAG probe 82 inside an opaque solid tip 83. Most of the intensityof the laser beam is absorbed within the opaque tip, and a smallfraction α_(v) P_(o) is absorbed by the Nd³⁺ ions in the Nd:YAG crystal.As the tip 83 gets hot, the magnitude of α_(v) increases, and theluminescence intensity generated at the Nd:YAG probe increasesaccordingly, this intensity being indicative of the probe temperature.The luminescence from the probe can be separated from the laserradiation from the Nd:YAG laser because most of it is emitted atwavelengths different from the 1.064 μm laser radiation. In practicethis is done by directing the probe luminescence, through coupler 12 andfiber segment 14 to photodetector 16A, which is made spectrallyselective to the probe luminescence by a multilayer dielectricinterference filter coated on the photodetector window. A photodetector17, made spectrally selective to the wavelength of 1.064 μm of theNd:YAG laser, monitors the backscatter laser radiation and is used forreferencing the luminescence intensity to the intensity of the laserradiation. The microprocessor processes the signals from bothphotodetectors into a signal indicative of the probe temperature. Whenthe highest desired temperature is reached, the microprocessor sends asignal to the laser power supply switching off the Nd:YAG laser.

It may be emphasized that the temperature measurement and control doesnot require the introduction into the patient's body of anything otherthan the laser radiation delivery fiber guide and the hot tip at the endof it.

12.0 Sequential Multiplexing and Transmission of Signals from ElectronicSensors on a Continuous Length of Optical Fiber.

The systems described above for the measurement of distributed forcesusing as a sensing probe a continuous length of an optical fiber are allbased on the use of at least two light-guiding regions in the opticalfiber, wherein a fraction of the intensity of the pulsed or AC-modulatedinterrogating light launched into one region is deflected into thesecond region. Several types of such fibers have been described andillustrated with appropriate figures, where the two light-guidingregions are labelled differently for different fiber types. For thepurposes of this application the light-guiding region into which theinterrogating light is launched is designated herein as region A,regardless of the type of fiber, and the light-guiding region into whicha fraction of the intensity of the interrogating light is deflected isdesignated as region B. In all of the above-described embodiments thelight deflected into region B at any point along the fiber is processedwithin region B into a light separable from the interrogating light andfrom light deflected at any other point by another force actingsimultaneously on the fiber. In the case of the optical fiber describedin section 3.5 the lights deflected into region B at different pointsare separated in the time domain in the transmission mode, by causingthem to arrive at the distal fiber end at different resolvable times.This fiber, like the ones in which luminescence conversion occurs inregion B, can be used for multiplexing the signals coupledsimultaneously into the fiber by numerous sensors or other devices. Inmany industrial application the signals may be coupled into the fibersequentially, rather than simultaneously, so one can use a simplerforce-sensing fiber, and the system must comprise means for instructingthe sensors (or other devices) to couple their signal into the fiber inthe proper sequence. Such a system is described below, with reference toFIG. 25.

Referring to FIG. 25, the fiber system comprises fibers F1 and F2spliced together at point P'. Both fibers F1 and F2 comprise a glasscore 1 with an index of refraction n₁ and a glass cladding 2 with anindex of refraction n₂ lower than n₁. Fiber F1 has a transparent coating3' around cladding 2, having an index of refraction n₃ not lower thann₂. Fiber F2 has a transparent second cladding 4 around cladding 2,having an index of refraction n₄ lower than n₂. Sensors S₁, S₂, S₃ andothers not shown are connected non-invasively to fiber F1 through theirphotodetectors P₁, P₂, P₃ and others. The non-invasive connectionconsists of a microbend on the fiber. The microbend deflects a fractionof the intensity of the interrogating light from light source LS out ofthe fiber core 1 into the cladding 2 of fiber F1. An index-matchingtransparent thermoplastic adhesive couples the deflected light to thephotodetector. The interrogating light is in the form of short lightpulses superposed on a continuous DC or AC beam. Each light pulseinstructs the sensor to `write` its signal output (for instance themagnitude of a pressure) on fiber F2 after a pre-selected interval t andfor a time Δt'. Each interval t is set so that a sensor `writes` itsinformation on fiber F2 after the preceding sensor has finished couplingits own information into the fiber. The `writing` consists of, forexample, a microbending force with a frequency which is a known functionof the value of the measurand. The microbending force is produced by apiezoelectric transducer PZT mechanically connected to a fibermicrobender M. The microbending force deflects a fraction of theintensity of the interrogating light out of the fiber core 1 intocladding 2, where it is trapped by total internal reflection from thesecond cladding 4 and transmitted to the photodetector PD. The opticalinterface between fiber F2 and photodetector PD can be so designed thatonly the cladding modes reach the photodetector, so as to eliminate orminimize the background noise from the core modes.

The sensors themselves are preferably, but not necessarily, siliconmicromachined sensors requiring only small electrical powers to operate,typically of the order of tens of microwatts. Thus they can be poweredby small batteries which, due to the low powers required, could giveyears of unattended service. Alternatively, the sensors could be poweredremotely by light source LS.

The electronic system which instructs each sensor to couple itsinformation to fiber F2 at a pre-set time is essentially alight-activated switch with a time delay for opening and closing,relative to the time of arrival of the light pulse at the photodetector.Such timed electronic switches are well known in the electronics field.

The system described with reference to FIG. 25 is an example of aso-called "hybrid" system, comprising electrical sensors and fiberoptictransmission of the sensor signals. A well designed hybrid systemcombines the advantages of already proven electrical sensor technologywith the immunity to electromagnetic interference of optical signaltransmission.

One advantageous feature of the system illustrated in FIG. 25 is thecapability of inter-sensor or inter-device communications. Light sourceLS can transmit any information to any device connected to the fiberthrough its photodetector, including commands to perform any functionfor which the sensor or device is suited. If, for example, it isrequired that sensor S₁ transmit the value of the physical parameter itis monitoring to the sensor S₃, then the microprocessor which processesthe photosignals from photodetector PD will feed back the informationfrom sensor S₁ to the power supply of the light source LS, and thisinformation will be converted into a modulation of the light output ofLS as it is transmitted in the code for sensor S₃. Thus any sensor ordevice in the multiplexed system can `talk` to any other sensor ordevice.

The advantages of sequential reading of the outputs of all the sensorsinclude the following:

1) All the sensors and/or other devices may share a relatively shortlength of conventional optical fiber (whether single mode or multimode)with no cross-talk problems;

2) The power requirements of the interrogating light source are greatlyreduced compared to the case if all the sensors had to transmit theirinformation to the photodetector PD simultaneously; and

3) The electro-optical system, including the information-processinghardware, is greatly simplified, as the signal from one sensor only isreceived by the photodetection system at any one time.

13. Visual Detection of Leaks of Cryogenic Fluids.

At the end of Section 3.1 I described a distributed fiber optic sensingsystem suitable for detecting leaks of cryogenic fluids. An alternateway of detecting leaks of cryogenic fluids is to paint the walls of thecontainer with a photoluminescent paint the luminescence color of which,when excited with ultraviolet radiation, is a sensitive function of thewall temperature. A leak of cryogenic fluid will decrease thetemperature of the wall nearest the location of the leak and thus changethe luminescence color at that point, thus revealing the leak. Suitablepaints include acrylic paints containing dissolved therein a mixture ofterbium and europium chelates, the terbium chelate characterized by agreen luminescence the efficiency (and hence, intensity) of which is lowat ambient temperatures and increases substantially with a decrease inthe paint temperature. Examples of terbium chelates showing thisbehavior are

terbium tris(1-phenyl, 1-3 butanedione)-1,10-phenanthroline and

terbium tris(1-phenyl, 4-trifluoromethyl 1-3 butanedione)-1,10phenathroline.

In these chelates each central terbium ion (Tb³⁺) is bound to threeligand molecules of the substituted butanedione and one ligand moleculeof 1,10 phenanthroline. The europium chelates are preferably made fromthe same ligands, differing only in the identity of the central ion.These europium chelates have an orange-red luminescence with anefficiency which is already high at ambient temperatures and does notincrease substantially with a decrease in temperature, or increases to amuch smaller extent than the increase of the efficiency of the terbiumluminescence. Thus, as the paint temperature decreases over a spot neara leak, the luminescence color changes according to the extent of thetemperature drop, from near orange-red to yellow-green.

14. A Fiber Optic Liquid Level Indicator.

The prior art liquid level indicators consist of a vertically disposedoptical fiber having at its lowest point a prism, bare fiber core orother component which, when relatively clean and in contact with air orother gases or vapors, allow the interrogating light to be transmittedto a photodetector, but interrupt said transmission when in contact withthe liquid. One disadvantage of these indicators is that the surface ofthese optical components has to be kept relatively clean, but iteventually becomes fouled up and interrupts the transmission of lighteven when not in contact with the liquid to be measured. The opticaltemperature probes of this invention allow the construction of a liquidlevel indicator free from this flaw. The indicator operates on theprinciple that a small temperature probe which absorbs light ofwavelengths a will be heated by the absorption of said light to asubstantially higher temperature when surrounded by air or any othergaseous medium than when immersed in a liquid at ambient temperature. Ifthe probe is photoluminescent and the luminescence intensity is asensitive function of temperature, as is the case with the probematerials discussed in section 2.1, then one can tell from the probetemperature under a given interrogating light intensity whether theprobe is or is not immersed in the liquid. The probe works best when thetemperature of the liquid is not higher than that of the air or vaporabove the liquid. This condition operates in the example of a preferredembodiment described below. The liquid container is a gasoline tank.

In operation, a small optical probe containing dissolved thereinluminescence centers chosen from the materials discussed in section 2.1and attached to one end of an optical fiber is placed in the liquidcontainer at the level at which the container is to be filled.Interrogating light of an intensity P_(o) and wavelengths at which theluminescence centers absorb a fraction αP_(o) of the interrogating lightwhich increases with increasing temperature is injected at the otherfiber end. The probe also contains a light-absorbing material in aconcentration sufficient to absorb most of the intensity of theinterrogating light. This intensity is chosen so that it increases thetemperature of the probe in air by an amount T at least several timesgreater than the minimum detectable temperature change. When gasoline isgradually pumped into the tank, its level continuously rises until itreaches the probe. As soon as the probe becomes immersed in thegasoline, its temperature drops to that of the gasoline. The temperaturedrop immediately alerts the operator (or the automated system, if suchis the case) that the tank has been filled to the desired level. Thetemperature drop produces a signal for the pump to stop its operation.

15. Fiberoptic Bus-Organized Systems for Data Communications and SensorData Acquisition.

Any sensor signals which are in the form of or can be converted intoacoustical or other mechanical forces could be coupled noninvasively tothe fibers discussed in section 3.5, having two light-guiding regionsdifferent of effective optical path length. For example, numerousexisting sensors and devices with a mechanical frequency output includeflowmeters and pumps, whose signals could be coupled to the subjectfiber through a fiber bending frequency proportional to the flow rate orthe rate of rotation of the pump impeller. Owing to recent advances inmicro-machined silicon sensors, and their suitability for constructing`intelligent` sensors, it can at least be argued that the mainattraction of fiber optics will increasingly lie in theelectrically-passive transmission of noninvasively coupled signals fromsensors and/or any other devices to a remote processing unit, ratherthan in the generation of the signals. It is already economically viableto incorporate just enough processing capability into a microchip sensorto convert the sensor signal into a pulse rate or frequency output. Thiscould drive an inexpensive microvibrator attached noninvasively to thefiber of this work, allowing the essentially error-free transmission toa remote station of the signal from that sensor and, simultaneously orin any arbitrary sequence, the signals from numerous other sensors socoupled to the fiber. Since the interrogating light pulse rate for a 1Km long fiber can be of the order of 10⁷ Hz, each vibration period willbe sampled by thousands or more interrogating light pulses, more thanenough to accurately reproduce the instantaneous sensor signal and tomeasure rapid signal changes in real time, including voice microphonesignals.

The fiber described in section 3.5, having two light-guiding regions ofdifferent optical path length, allows the real time collection, storingand integration of the electrical signals produced by each and allphotons or group of photons per signal pulse arriving at thephotodetector from each force-sensing point along the fiber. Since thefiber is so constructed as to automatically demultiplex and time-resolvethe optical signals from each sensing point, these signals, and theirtime evolution, could all be captured, stored and integrated as atwo-dimensional distribution of electrical charges in a scan converteror any other image storage tube or solid CCD array, each resolvable spotin the two-dimensional array representing the time-integrated chargewithin a `TV` frame.

An electro-optical system allowing said real time signal collection,storing and integration is illustrated in FIG. 26. Each horizontal linecontains information on the light intensity generated at a given instantfrom each spatially resolvable element of the fiber, and the location ofeach all sensing points. Each point could include the signals fromhundreds, or thousands, or more interrogating light pulses, depending onthe pulse repetition frequency (PRF) of the interrogating light. The PRFcan be any chosen frequency up to the order of about 10⁷ Hz. Eachvertical line represents the intensity changes as a function of time foreach corresponding, fixed point along the fiber. For example, the rotorof a turbine flow meter or pump will produce an oscillatory signal asillustrated in FIG. 26, the period of oscillation being indicative ofthe flow rate. Another vertical scan line could reproduce human speechpicked up by a voice microphone. The number of `TV` frames per second(the refresh rate) could be adjusted at will, so that each resolvablespot in the two-dimensional charge distribution can be the sum ofnumerous pulse signals.

16. An Infrared Image Converter.

The teachings of this invention can be applied to the construction of asensitive infrared-to-visible image converter. It was shown in section2.1 that the absorption of light of photon energy lower than the energyof a luminescent level of a material is strongly temperature-dependent.This fact can be used as a basis for constructing sensitiveinfrared-to-visible image converters, especially at liquid heliumtemperatures. A probe which absorbs infrared radiation undergoes atemperature increase. Referring to FIG. 1 and equation (5) of section2.1, it can be noticed that for any value of (E_(v) kT) the temperaturecoefficient of the luminescence intensity I_(f) increases as the initialabsolute temperature decreases. The relative increase ΔI_(f) in theluminescence intensity follows the relation

    ΔI.sub.f /I.sub.o =(E.sub.v /kT.sup.2)ΔT

    or

    ΔI.sub.f /I.sub.o =(E.sub.v /kT.sup.2)(H/C.sub.v)

where H is the heat generated by the absorbed infrared radiation andC_(v) is the specific heat of the probe. It is known that the specificheat of essentially all materials is orders of magnitude smaller atliquid helium temperatures than at ordinary temperatures. Thus, if theprobe is made thin to reduce its thermal mass, a relatively lowintensity of infrared radiation can be converted into a substantialincrease of the intensity of the fluorescence light emitted by theprobe.

A preferred embodiment of an infrared image converter according to thisinvention is illustrated in FIG. 27. The probe is a two-dimensionaltri-layered thin plastic film PF doped with a fluorescent dye chosenfrom the group of plastic-soluble dyes including red- orinfrared-fluorescing porphyrins, phthalocyanins, violanthrone,isoviolanthrone and their derivatives. The total thickness of the filmis smaller than 500 682 m, and preferably smaller than 50 μm. Layer Ahas an index or refraction n₁ and contains the dye dissolved therein.Layer B is clear and has an index of refraction n₂ substantially lowerthan n₁, and layer C is an infrared absorbing, visible reflecting thinmetal film like nickel. The film is exposed to the infrared image to beconverted through lens D. Interrogating light of wavelength λ_(v) isinjected into layer A from the edges or by a prism coupler (not shown).Referring to FIG. 1, the interrogating light has a photon energy equalto (E_(s) -E_(v)). Referring again to FIG. 27, the interrogating lightwill propagate along film layer A and, at each point, atemperature-dependent fraction α of its intensity will be absorbed andwill generate fluorescence light with an intensity proportional to α,the magnitude of α and the intensity of the fluorescence increasing withincreasing temperature according to equation (3) of section 2.1. Thus,an infrared image focused on the film will generate an image having thewavelengths of the fluorescent dye. This wavelengths are well within thesensitivity range of current television camera sensors. Thus, byfocusing the fluorescent image into a TV camera, the image can bedisplayed on an ordinary television screen.

Since changes may be made in the foregoing disclosure without departingfrom the scope of the invention herein involved, it is intended that allmatter contained in the above description and depicted in theaccompanying drawings be construed in an illustrative and not in alimiting case.

I claim:
 1. An arrangement for selectively generating laser radiationwithin a defined spectral band, comprising(a) an optical fiber having acore with an index of refraction n₁, a radius not greater than a fewmicrometers, and doped along its length with laser material socharacterized that, when excited with optical radiation of wavelengthsλ_(s) within an absorption band characteristic of said material, itemits luminescence radiation within at least two spectral bands, thefirst of said bands including wavelengths λ_(f1) and the second of saidbands being said defined spectral band and including wavelengths λ_(f2)different from λ_(f1), the material being further characterized byemitting laser radiation within said first band including wavelengthsλ_(f1) when pumped with sufficiently high intensities of said radiationof wavelengths λ_(s) in a suitable configuration, the material beingadditionally characterized by emitting laser radiation within saidsecond band including said wavelengths λ_(f2) when pumped withsufficiently high intensities of said radiation of wavelengths λ_(s) ina suitable configuration, the fiber having a first end and a second end;and (b) a second material characterized by absorbing light of saidwavelengths λ_(f1), said second material being in optical communicationwith said laser material in such a manner as to absorb radiation emittedfrom said laser material within said first band of wavelengths λ_(f1)along the fiber length and thus selectively prevent laser action atthese wavelengths without preventing laser action at said definedspectral band including wavelengths λ_(f2).
 2. An arrangement as claimedin claim 1 wherein said laser material includes trivalent neodymium ions(Nd³⁺) incorporated within the core of said fiber, and said secondmaterial includes trivalent samarium ions (Sm³⁺) in a first claddingcontinuous to and in contact with said core.
 3. An arrangement asclaimed in claim 2 wherein said first cladding in contact with said corehas an index of refraction n₂ lower than n₁.
 4. An arrangement asclaimed in claim 3 wherein said core and said first cladding aredisposed within a second cladding having a cross-sectional area which,taken transverse to the fiber axis, has a first path across said areawhich passes substantially through the geometric center thereof whoselength is substantially different from the length of a second pathacross said area which passes substantially through the geometric centerthereof, said second path being substantially perpendicular to saidfirst path.
 5. An arrangement as claimed in claim 3 and additionallyincluding a source of laser pump radiation of said wavelengths λ_(s)launched into the first end of the fiber.
 6. An arrangement as claimedin claim 4 and additionally including a source of laser pump radiationof said wavelengths λ_(s) launched into the first end of the fiber.
 7. Asource of stimulated optical radiation of wavelengths within the band ofluminescence wavelengths of trivalent neodymium ions includingwavelengths λ_(f2) near 940 nanometers, comprising the arrangementclaimed in claim 5 wherein said source of laser pump radiation of saidwavelengths λ_(s) is sufficiently intense to generate a populationinversion of said trivalent neodymium ions and light amplification ofsaid wavelengths λ_(f2) along the length of said fiber.
 8. A source ofstimulated optical radiation of wavelengths within the band ofluminescence wavelengths of trivalent neodymium ions includingwavelengths λ_(f2) near 940 nanometers, comprising the arrangementclaimed in claim 6 wherein said source of laser pump radiation of saidwavelengths λ_(s) is sufficiently intense to generate a populationinversion of said trivalent eodymium ions and light amplification ofsaid wavelengths λ_(f2) along the length of said fiber.
 9. A source ofstimulated optical radiation as claimed in claim 8 wherein said sourceof laser pump radiation of said wavelengths λ_(s) is a semiconductorlaser diode array.
 10. An arrangement as claimed in claim 3 wherein saidlaser material is co-doped with trivalent ytterbium ions (Yb³⁺).
 11. Anarrangement for pumping an erbium-doped fiber laser or amplifier,comprising a source of stimulated optical radiation as claimed in claim7 and a second fiber having a first end and a second end, the first endbeing optically connected to the second end (the output end) of thefiber of claim 7, said second fiber including a core doped withtrivalent ytterbium ions (Yb³⁺).
 12. An arrangement for pumping anerbium-doped fiber laser or amplifier, comprising a source of stimulatedoptical radiation as claimed in claim 8 and a second fiber having afirst end and a second end, the first end being optically connected tothe second end (the output end) of the fiber of claim 8, said secondfiber including a core doped with trivalent ytterbium ions (Yb³⁺).